Talk:G-force/Archive 3

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The phrase "non gravitational acceleration"

Wolfkeeper, I agree with your edits and philosophy in this article, but I think you should steer clear of the term "non gravitational acceleration", since I think most readers would be misled by it. For example, say I have an accelerometer sitting on the ground. I can see a reader thinking, "Well, it has no acceleration whatsoever, so it certainly doesn't have any non-gravitational acceleration!" There are plenty of clearer alternatives, like "acceleration relative to free-fall", or "acceleration, added to the quantity called 'acceleration due to gravity', which is 9.8 m/s^2 on the Earth's surface". (OK those could use improvement but I hope you see what I'm getting at.) Agreed? :-) --Steve (talk) 03:01, 22 January 2009 (UTC)

Nice point. The phrase at it stands seems... wrong, although I think I do understand the arguments made in the section above. --John (talk) 03:12, 22 January 2009 (UTC)

Please see also my post here. --John (talk) 03:14, 22 January 2009 (UTC)

This phrase (non-gravitational acceleration) has caused some problems, but perhaps it should, as it's actually accurate. The accelerometer essentially keeps track of the non-gravitational part of the acceleration (actually, the non-gravitational part of the forces) on an object. Gravitational forces can't be measured (or even felt) because they "act" on every part of a body at once, so there's no stress or strain to tell you that they're "there" or not. Which is just as well, because in Einstein's view they don't exist at all.

But we all feel the force of gravity, don't we? Actually, no. What we FEEL is the reaction forces, which are mechanical, and start at once place in our body (such as the feet) and move upward as a series of stresses. But stictly speaking, this is all non-gravitational, and that's why it all works just the same when you're in a rotating space station or a rocket, and it's the floor pushing up on you which takes the "place" of the "gravity force." Actually, these inertial forces are taking the place of the mechanical forces which are reacting to the "gravity forces," (I use quotes because of the suspect nature of "gravity forces" and the fact that they exist only for Newton). So they're all very familiar, but they're all mechanical-- caused by one atom pushing on another by chemical bonds, and so on. You never feel gravity per se at all, and neither do your instruments like spring scales and accelerometers. All you feel is these mechanical counter-reactions, which are elastic stresses and strains and compressions, etc., transmitted through the body.

. Now, a word about accelerations, which are used very loosely, but shouldn't be. If a thing isn't accelerating in our chosen frame, then it isn't, period. You can talk about its "acceleration relative to free fall," (that is, relative to an inertial frame), but that means you're talking about an accelerated frame, such as a hotel room, or a room in a rocket in space which is blasting so that everything is stuck down by inertial forces. But in both of those cases, everything in the room clearly feels only one force (the mechanical one pushing up through your feet) and you ASSUME the other one (the gravitaional force of the earth, or the inertial force of the accelerated frame in a rocket, caused by the rocket motor) only because you know that forces need to be balanced when things aren't in motion, and in your frame, things are not moving. Thus, everything you see which is not moving in your frame, you presume to have (in Newton's view) two forces on it which balance each other; but only one of which you can measure, and the other of which you assume to exist, only because things have no motion (in your frame-- again, we're taking about things like a glass of water or an accelerometer on a table, in a hotel room on earth, or in a room in a rocket). So a thing on a runway has no acceleration (net, gravitational or otherwise), but we presume that it has two balanced types of forces: one gravitational and one mechanical (acting up from the ground). It's useful to view these Newtonianly as two balanced forces, but it's not useful (I find) to view them as two balanced "accelerations." But you can, if you like.

Now, accelerometers, like spring scales, don't really measure accelerations directly, but rather measure forces. Accelerations are assumed from knowing the mass of the parts. And the forces we can measure (with both the spring scale and the accelerometer) are mechanical ones, NOT gravitational ones. They are caused by the electromagnetic force of the fundamental forces, not by gravity. The gravity force isn't felt, and not measured.

That's why we're at pains to say that accelerometers only measure NON-gravitational forces (and non gravitatinal accelerations). When you think you're using them to measure gravity, all you're REALLY doing is measuring the mechanical counter-reaction, and assuming that gravitational forces/accelerations are the same, in very special situations where you know they must be.

Of course, the general fact may or may not be true, and usually isn't. Usually, what the gravitational field is in the vertical direction is, is an unknown, and must be "input" from an outside source, like a lookup table. For an accelerometer sitting quietly and motionlessly on a bench or in an instrument panel on a run-way on Earth, it's a good bet that the mechanical counterforce it's measuring (caused by the pressure of the mountings, transmitted through the body of the device to some sensitive element like a crystal inside), is the SAME as the gravitational force/acceleration acting on the thing. It has to be, because they must balance out if the thing isn't moving up and down, and we can SEE that it isn't (this becomes a problem when we don't know if it is or isn't, as in an airplane in clouds at night). In such cases, it's said that the accelerometer is measuring the acceleration of gravity, but all it's doing is measuring the acceleration of the counterforce, and the equality of the gravitational "force" or acceleration, is assumed. In fact, you really can't usually assume such things. Inside a blasting rocket, for example, there is no gravity and the "counterforce" (the force from the instrument mounting) is all there is. Sitting on a bench or runway, the device says "+1 g acceleration, upward." In the Newtonian view, you see it's not going upward, so you assume that the +1g force/acceleration it's measuring is being counteracted by a -1 g gravity field, acting downward. But the device is not measuring gravity or gravity force, it's measuring the electromagnetic counter-force.

The reality of this is apparent when you get away from places where you know what your position and "actual acceleration" (relative to some reference) really are. In such situations, you find that the accelerometer really only measures the non-gravitational component, and NO gravitional one. For example, if you're in a spaceship falling into a black hole at 10 g's, and also blasting sideways (or any direction) at 0.2 g, all your accelerometer will read is "0.2 g." It's not measuring the field you're falling into, but only measuring the inertial force/acceleration created by your rocket engine. You, in your rocket seat, may feel a sort of inertial force that feels like gravity and pulls you toward the floor, but your faithful accelerometer doesn't see it that way. It's bolted to an instrument board and held fixed relative to the rocket engine, and so all pushes by the rocket engine are transmitted to it directly, where they are read simply as non-gravitational forces/accelerations, period. Very simple.

In an airplane the same thing happens. The accelerometer only measures the sum of all the mechanical (non gravity) forces/accelerations on it. For sideways motions these are straightforward, but for up-and-down, you no longer have the luxury of presuming the device measures gravitational acceleration indirectly. All it measures is the total up and down reaction forces/accelerations, and you must tell it by hand what fraction of these are due to the Earth's g-field, so that it can presume the rest are caused by mechanical positive and negative lift forces (which are analogous to the pressure of the runway against the wheels of the plane, but now can be less or more than the plane's weight, and less or more than local g). It is only these last which contribute to vertical position (change in altitude) when integrated. At every altitude and time you must "tell" the device how heavy it should be if it weren't moving up and down, and then it must measure how heavy it actually is (mechanical acceleration) and take the difference to get vertical inertial acceleration which must be paid attention to, as it's a marker for changes in vertical velocity and position. Again, the accelerometer is not summing gravitational acceleration and vertical mechanical acceleration. Rather, it feels only the latter, and totally neglects the former. SBHarris 08:43, 22 January 2009 (UTC)

This phrase (non-gravitational acceleration) has caused some problems, but perhaps it should, as it's actually accurate.

No, it shouldn't. While accurate, more than 99% of readers would misunderstand it. Please read the point 5 of WP:NOT PAPERS. -- Army1987 – Deeds, not words. 12:29, 22 January 2009 (UTC)

I'm with Army1987. Sbharris, if you read what I wrote originally , you'll see that I was not arguing that the phrase "non-gravitational acceleration" is incorrect. Quite the contrary, I already 100% agree that it's correct, and I'm sorry that you spent so much time typing the above text to defend its correctness. My point was that the phrase is liable to be misinterpreted by the average reader, and therefore should be avoided. There are plenty of other words in the English language, we can find another way to say it that is equally accurate but that every reader will understand. --Steve (talk) 17:12, 22 January 2009 (UTC)

Well, how about saying an accelerometer measures the component of acceleration which causes mechanical stress? That's what you feel with your butt against a chair, as in seat of the pants flying. And the reason for this, is that inside every accelerometer, there's a thing which corresponds to your butt, and another that corresponds to the chair, and something that measures the interaction (pressure/stress) between them. So an accelerometer, in a way, is just a mechanical ass with an electrical output. It doesn't measure gravity directly anymore than your butt does. SBHarris 22:40, 22 January 2009 (UTC)

I think this suggestion has the same problem as before. Someone standing still has total acceleration zero. If that someone is a bright physicist like you or me, they will think, the acceleration vector, namely 0, has two components, one is gravity, and the other is the equal and opposite acceleration due to the ground. So the "component of acceleration that causes mechanical stress" is the latter, the acceleration due to the ground. BUT, the average reader isn't a bright physicist like you or me. They will think, "if the total acceleration vector is zero, then it's the null vector, and I know that the null vector has no nonzero vector components. Its x-component is zero, its y-component is zero, and I guess its "component that causes mechanical stress", whatever that is, is zero. So I guess the g-force is zero." Can we keep trying to come up with something better that these readers will be able to easily grasp? I appreciate that you're coming up with concrete suggestions. --Steve (talk) 21:57, 23 January 2009 (UTC)

Kudos

I was stopping here to say "kudos" for this article. It is a very clear explanation of how accelerometers work that I could not find elsewhere. I really liked the part where acceleration is explained as km/hr per second. That makes it SO MUCH CLEARER for my students. Good work. Wikipedia is quite a resource. —Preceding unsigned comment added by 66.108.31.53 (talk) 03:14, 22 January 2009 (UTC)

Are you being sarcastic? Somebody has stripped the article of all references to accelerometers in any case. SBHarris 03:21, 22 January 2009 (UTC)

Trying to get past the term "gravitational acceleration"

The term "gravitational acceleration" can mean 2 things. It can mean the acceleration (relative to an inertial reference frame) that a physical object actually undergoes in response to gravity, for example when it's in free-fall. Obviously this quantity isn't measured by an accelerometer, since accelerometers read "0" in free-fall. "Gravitational acceleration" can also mean the quantity g, 9.8 m/s^2 at the earth's surface. Obviously this quantity can be measured by an accelerometer, for example by placing it on the ground and reading the dial.

No. The reading on the accelerometer on this case is merely a reading of the force the table (or whatever it's sitting on) is exerting on the accelerometer. It's mistaken for g because it's the same number as g, if the thing happens to be sitting still--- but the device is not actually measuring g. Demo: replace the legs of the table with 4 rockets that keep the table hovering at constant height. The reading on the dial doesn't change. Now (the magician trick), remove the Earth and its gravity field. The reading still doesn't change. Why? Because it never had anything to do with the gravity field. It just happened to be the same number, because you (perhaps inadvertantly) calibrated it by fixing up your support system's force, so the accelerometer didn't move up or down, when the Earth was there. But the force of the support system is what is (always) being measured. Further demo of this: remove the support, and the dial goes to zero, whether the Earth is there or not. SBHarris 06:06, 22 January 2009 (UTC)

I measure my height by marking the top of my head with a pencil on the wall, then measuring with a tape-measure how high the pencil mark is. Does that mean the tape measure isn't actually measuring my height? Well, yes and no, but more to the point, who cares? The real point is that with a bit of effort we can think of creative alternatives that are completely unambiguous, and the first step is to collectively abandon phrases like "measures / does not measure gravitational acceleration" that a reader can interpret in more than one way. I mean, if we're able to spend hours debating how to interpret this particular phrase, a non-physicist reader in a hurry has no hope at all. :-) --Steve (talk) 08:04, 22 January 2009 (UTC)

Well then, as I said, just define "g-force" as the accelerational force you feel (or anything feels) when you (or it) is accelerated by being shoved (or pulled) by another material object. That's simple, concrete, and exactly true. It leaves out acceleration due to gravity (which is non material), but you don't get any g-force from that, anyway (and don't feel any, and your accelerometer doesn't feel any, either), so that's all cool. How about it?

Jump from a plane: no g-force, you're in free fall. But very soon, you start to feel g-force as air resistance builds, and 5 seconds later when you hit terminal velocity in skydiving, the g-force is now 1 g. You're being shoved by air, and it feels like lying on a cushion of air. Then, your 'chute opens and you feel several g's as the pull of the risers accelerates you upward (even as you're still actually traveling downward). See how it works? All these g-forces are always being caused by shoves or pulls from real, material stuff. Gravity and its pulls never enters in, except for the annoying pushing it takes to keep you from falling to the center of the Earth. But that's a push from material stuff, too, and easy to identify. Your bathroom scale provides it, for example. So again, gravity isn't part of the picture of g-force, except for the pulls and pushes needed to resist it. SBHarris 11:47, 23 January 2009 (UTC)

Sbharris, please stop explaining things to me which I already understand and agree with. What I'm saying is that people don't intuitively think of themselves as being accelerated by the floor, especially when their actual acceleration vector is zero. You can spend all day on this talk page chiding people for not thinking of themselves as being accelerated by the floor, but at the end of the day, if these people are misled and confused by the first sentence, then that sentence is a failure. It doesn't matter whether it's technically correct or not.

Nothing you say on this page will change the fact that an average reader standing on the floor will not think that they are undergoing any sort of acceleration, gravity or not, unless we tell them so explicitly and with examples, as is done later in the article, but which I don't think can be done in the first sentence. --Steve (talk) 21:36, 23 January 2009 (UTC)

As far as I can tell, Wolfkeeper has using the first definition and others have been using the second one, and y'all have been arguing past each other on and on. So instead, maybe we can agree to not use the term "gravitational acceleration" in the article (at least not without additional clarification) and put your time instead towards finding a different, creative and concise and unambiguous phrasing. Maybe a starting point could be:

"g-force is a measure of an object's acceleration relative to free-fall. For example, an object sitting on the ground is stationary, but is accelerating at 1g relative to how it would be moving in free fall, so its g-force is 1. If it's accelerating upwards at 1g relative to the earth, it's accelerating upwards at 2g relative to free-fall, so its g-force is 2. In space, there is no free-fall acceleration, so the g-force measures acceleration directly. More generally, g-force is the vector sum of [blah]".

A more concise one, maybe for the intro: "g-force measures the total effective force that one feels due to the combined effects of gravity and acceleration". --Steve (talk) 03:25, 22 January 2009 (UTC)

• Thanks Steve. That helps. Greg L (talk) 03:45, 22 January 2009 (UTC)
• That's likely more clear than my attempts above, however for the "average" reader, I'd prefer to see something more accessible in the intro. Something along the lines of "remember when your Dad used to swing you around?" or "remember when you first had a car and took that corner in the gravel and got thrown across the entire front seat?". Not quite that corny, but is there a way to convey a human experience first, before delving into the theoretical and technical details? Franamax (talk) 04:00, 22 January 2009 (UTC)

• I would have thought that what is currently in Nature of the measure, which talks about stopping and turning in a car, ought to be pretty familiar to anyone in the English-speaking world—even a very young child. Moreover, that section now makes the measure more accessible by expressing it in terms that are far, far easier to comprehend than “9.80665 m/s²”, it introduces the measure by telling how that complex-looking thing can be expressed as 35 km/hr per second or 22 mph per second. I can’t imagine how the subject of acceleration be made any simpler than that. Greg L (talk) 05:16, 22 January 2009 (UTC)

Well yes, but at the same time that is a profoundly exclusionary statement, since it cuts out all of those who don't ride in cars, and the greater subset who don't see a speedometer on a regular basis. Point taken however. Franamax (talk) 07:13, 22 January 2009 (UTC)

Greg, I'm on a global interpretation bent right now, wrestling with Canada Census Ethnic Origin data, accuracy, presentation and interpretation - so please bear with me when I insist there should be something for the truly average global reader. :) Franamax (talk) 07:13, 22 January 2009 (UTC)

The article currently reads: "The measurement of g-force (or g-load) is the measure of an object's acceleration—gravitational and inertial."

Now me reading that would naturally assume that you add the inertial acceleration to the gravitational acceleration. I would also assume that there's only one way to measure an object's acceleration. I mean otherwise it would have said that, right? In English, you try to communicate by saying things that the other person probably doesn't know about. "I saw Fred the other day with that woman". But you wouldn't say that in anything but a sarcastic sense if the woman you saw him with was his happily married wife.

Here, adding the two accelerations is exactly what you don't do: you SUBTRACT the gravitational acceleration from the inertial. Now the argument has been made more or less, that we shouldn't try to 'confuse' readers by bothering them with 'unnecessary' details like that, but all I see is an entire article that probably overall explains the topic in such a way that it cannot be correctly understood. I think we need to credit the readers with some intelligence, give the accurate definition at least somewhere, and give a few good examples (and there's already some in the article).- (User) Wolfkeeper (Talk) 14:34, 22 January 2009 (UTC)

OK, how about "g-force measures the total effective 'force' that one feels due to the combined effects of gravity and acceleration. The g-force is defined to be "1 g" for a stationary object subject to gravity on the earth's surface, and would be "0 g" in any 'weightless' environment such as free-fall or an orbiting satellite, and can be greater than "1 g" in a rapidly-accelerating rocket, for example. More precisely, if a is an object's acceleration vector, and g is the gravitational acceleration vector (pointing towards the center of the earth), then the g-force for that object is |a-g|/(9.8 m/s^2) g's." --Steve (talk) 17:26, 22 January 2009 (UTC)

• F = ma applies to any object accelerated relative to a given frame of reference—usually you. If you want to stand on the earth and watch something drop, then force F in this case is gravity (without the opposing equal force of the ground pushing up) and the object accelerates relative to you. If you are falling with the object, it is stationary with respect to you and the only way to get the object to accelerate relative to that frame of reference is to apply some other force to accelerate the object.

  These are nuances that are well beyond the scope of this article. Readers can click on the links to Newton’s laws of motion to learn more. This article makes it clear that gravity is a 1 g force and inertial accelerations with respect to the earth add or subtract to this ever-present force. Even still, this article speaks to the issue of free-falling bodies (like amusement park rides) to ensure readers have all the important basics down. Greg L (talk) 22:08, 22 January 2009 (UTC)

• The "G-force" is ordinarily used only to quantify what a person "feels" (or would feel if there was a person there). Thus, a person falling into a black hole at 1000g's feels no G-force. Ironically, what an accelerometer measures is not total acceleration, but only that part of acceleration which a human would feel, and the part that causes g-stress, or g-load, or otherwise breaks stuff. Accelerometer output is a perfect proxy for what a human riding with the accelerometer would feel; it gives a quantitative output of that. So acclerometers don't measure acceleration, they measure stress due to acceleration. They measure the acceleration which can be felt. This is not surprising, because the instrument isn't magic, and it can't measure (given a suitably large efffect) anything a human cannot feel. The seat of your pants is a crude accelerometer, and the accelerometer only gives you quantitatively what your butt tells you qualitatively. SBHarris 22:29, 22 January 2009 (UTC)

Acceleration and forces --- disputed!

I hate to open another debate on this page, but there's some material in the article that's misleading.

If I place a book on a table, the force of gravity on a book and the normal force that the table exerts on the book are not an action-reaction pair. (I mentioned this in passing above, but didn't realize it was in the article as well.) They are equal and opposite force, but both act on the same object, and have a different fundamental nature (gravitational and electromagnetic respectively.)

Newton's third law is all about interactions; that when object A acts on object B, object B acts on A as well. It's stating that every force is really part of a mutual interaction between two objects. In the example above, as the earth acts on the book, there is also a gravitational force acting on the earth. And as the table exerts a normal force on the book, the book exerts a normal force on the table.

I know this sounds a little pedantic, but as a TA I have enough trouble explaining this to students after they miss this on a test, without Wikipedia misleading them... :) --Starwed (talk) 08:28, 22 January 2009 (UTC)

Yes, the weight of the book is an action which acts on the table and it is in the reaction pair with the normal force of the table that acts on the book.- (User) Wolfkeeper (Talk) 09:36, 22 January 2009 (UTC)

• Welcome to Wikipedia. Yes, there is a lot of material in this article which is misleading and just plain wrong. The idea that accelerometers “measure” gravitational acceleration, for example, is wrong. You’ll see some nonsense that the reading of an accelerometer in an airplane sitting on a runway indicating +1 g (upward) is due to the Earths’ gravity somehow accelerating the sitting airplane “upward in spacetime” (as though spacetime had a direction we could call “up”), and the accelerometer is supposed to know this and somehow measure it (actually, the accelerometer merely measures a force on a load crystal which it interprets as an upward inertial acceleration, which it would be, in absence of a gravity field, and outputs that because it doesn’t “notice” gravity). All this would be funny, but one editor is insisting on this kind of stuff and I suspect he’ll eventually simply have to be suppressed by force. Like I said, welcome to Wikipedia.

  As for the rest, it would help if you’d point out exactly the parts you’re having trouble with, or better yet, just go in there and change them, so they read right to you. I’m eventually going to have to do that myself, as soon as I get done with this TALK page discussion about all this (go there and have a look).

  For certain, Newton’s third law applies only to single force of a given type, so you have to have two separate vectors, each with two ends/heads. The attractive one is for the gravitational force on a book (let’s pretend that exists, ala Newton) and repelling one is for the “electromagnetic” normal force, too (the one that supports the book against the table it sits on). I like to write attractive force vectors like this <--- ---> and repelling ones like this --- > < ---

  They both exist, side by side for the book, the attractive one for gravity and the repelling one for the normal force. Let’s turn gravity on its side for a moment because the vectors are easier to draw: we’ll put the table and the Earth on the left, and book on the right. The vector arrows for each force are equal for the 3rd law, and there are two of them, one for each force. The same length gives us the 3rd law constraint that the normal force vector pushes as much on one end (the book) as it does on the other (the table), and also the same is true for the gravity vector that pulls two things together.

<--- --->

TABLE --- > < --- BOOK

What confuses students is what you then do, after getting done with the third law, is isolate the book as a free-body, and then it’s permissible to draw “half-vector” forces to it. There’s a gravitational force that pulls the book toward the table. There’s another equal force (not action/reaction pair, but equal or else the book would be in motion) which is the normal force pushing the book upward:

Normal force --- > BOOK <-- Gravity

These forces sum to zero so the book doesn’t move, and that’s Newton’s second law.

In the relativistic view, things are more complicated and only the normal force exists. It causes an acceleration off the 4-D geodesic path, where ordinarily the object would fall and thus undergo time dilation, and have its proper time (the time that passes for it) maximized for the world-line it follows. Instead, by being accelerated by the normal force, it takes another path. But the point is that the unbalanced force still causes a sort of acceleration from the geodesic in 4-D, even though it’s difficult to draw in 3-D.SBHarris 10:19, 22 January 2009 (UTC)

Wolfkeeper, put a bowl containing some water on a scale. Then, take a stick and partially immerse it into water, holding the other end still in your hand above the water's surface. Do you expect the reading of the scale to change, and why? Now, perform the experiment; if the result is not what you expected, try to figure out why. If you think you've got it, try to predict what would happen if you used another stick with similar size but very different density. Then, actually perform the experiment. Was the result the one you expected? (Hint: Starwed is completely right.) -- Army1987 – Deeds, not words. 12:36, 22 January 2009 (UTC)

Buoyancy and Newton's third law; I don't even need to do the experiment. ktxbai. (p.s. not net buoyancy, so it's independent of density) Yes, he's completely right, never said otherwise.- (User) Wolfkeeper (Talk) 12:53, 22 January 2009 (UTC)

I had misunderstood your post, then. Now that I've read it more carefully I understand what you meant. -- Army1987 – Deeds, not words. 14:14, 22 January 2009 (UTC)

GOODDEF

I agree that the article lead kinda, sorta, almost defines what g-force is, in a way, however WP:NOTDICDEF states:

articles should begin with a good definition and description of one topic

WP:GOODDEF states:

"A definition aims to describe or delimit the meaning of some term (a word or a phrase) by giving a statement of essential properties or distinguishing characteristics of the concept, entity, or kind of entity, denoted by that term." (Definition)

A good definition is not circular, a one-word synonym or a near synonym, over broad or over narrow, ambiguous, figurative, or obscure. See also Fallacies of definition.

In other words, by (as it turns out very old, good) policy you are not allowed to be very vague on what this is just to make it read well or if you are concerned about 'confusing people'. The way to not confuse people is just to explain it really well. This is also what the WP:LEAD guideline says.- (User) Wolfkeeper (Talk) 09:32, 22 January 2009 (UTC)

Edit warring

Only time for a quick comment, having come here from WP:AN3.

1. Stop edit warring or you'll get blocked :-) But I think this has mostly happened. Good.
2. Looking at [1], both sides appear wrong. As has been said above, gravity is acceleration and accelerometers measure acceleration. IMHO mentionning gravity at all in the lead just confuses. Just say "The measurement of g-force (or g-load) is the measure of an object's acceleration" and stop there.

William M. Connolley (talk) 14:42, 22 January 2009 (UTC)

I've closed the AN3 thread with a result of "peace". This had better be true William M. Connolley (talk) 19:01, 22 January 2009 (UTC)

Some edit warring is inevitable is one party has some wrong idea about something as clear as the function of a basic instrument of physics, and keeps pushing it. Wikipedia really doeesn't have any good answers for what to do, then, as no consensus will ever be reached.

The article on G-force (this one) might be improved (or the fighting stopped) if all mention of accelerometers was removed, but the fight is inevitable on the Accelerometer wiki.

Accelerometers ONLY measure only some kinds of acceleration. They do not "see" the acceleration produced by gravity. Thus, an accelerometer dropped in a g-field, which is surely accelerating, will read "zero," just the same as if it was out in space, floating. In the same way, an accelerometer held hovering over the Earth with a rocket, will read +1g, but that figure will not change if the Earth is then removed, along with its gravitational field. ERGO, the accelerometer never saw that field, or noticed the presense of the Earth, either.

Ironically, what accelerometers DO measure, is the total non-gravitational acceleration on an object, and THAT is what a person feels, and THAT is what is measured in units of "g-force." So that's really the reason probably why the instrument creeps back into this acticle, although nobody has clearly articulated that, yet. For example, an astronaut in orbit is still well within the Earth's g-field, but feels no g-force (absent micro-tides) and his accelerometer agrees with him, and also reads "zero-g". So this instrument is a convenient way to measure the thing that we'd like to quantify in this article. The problem is that this article is all screwed up, and nobody has yet defined it very well. The "G-force" is the mechanical thing-- it is the sum total of all mechanical forces on a body, and does not include gravitation (really, it doesn't). If it's zero, you float; it doesn't matter what gravitational field you're in. If the g-force on you is high (for whatever reason, whether or not your being squashed by Jupiter or squashed by being shot from a cannon) then the g-force on you is high, and your accelerometer in your pocket will tell you exactly how high. It's the perfect readout for your g-force stress. "But wait," you said, "you mentioned Jupiter." So I did. But if you're falling into Jupiter, you have no stress (and your accelerometer reads zero and you feel weightless). BUT if you're held above Jupiter in a balloon, the stress on you is not from Jupiter, it's from the platform of the balloon which is trying to keep you from falling. The G-force you feel is from the floor of the balloon cabin, and that's the force your accelerometer feels as well. And will read out as an "acceleration." But it's only the "accleration a human would feel." It's not the sum total of every acceleration you can think of. SBHarris 22:03, 22 January 2009 (UTC)

No, that's completely incorrect - see Equivalence principle. Being in a gravitational field of 1 g cannot be distinguished from accelerating at a constant 1 g by an observer (including an accelerometer). Note that in your example above, an accelerometer being held stationary above the Earth in a rocket is still not accelerating. Hal peridol (talk) 23:25, 22 January 2009 (UTC)

Help me out. I don't see a single thing you've said which contradicts anything I said. Great name, BTW. Are you some physicist we know, who's gone off their Haloperidol? If you're a sock, beware. SBHarris 23:46, 22 January 2009 (UTC)

Hi, Sbharris. We seem to be talking at cross purposes here. The acceleration you feel (g-force) *is* the sum of all accelerations. As in your example, if you are in a motionless (hovering, or landed, as they are equivalent) rocket in a gravitational field, you feel the same as if you are in a rocket that is accelerating. Likewise, if you are in a rocket accelerating upward at 1 g from the Earth, you feel twice as heavy, as you are adding the gravity, which is equivalent to an acceleration upwards at 1 g. Hal peridol (talk) 00:28, 23 January 2009 (UTC)

How about "accelerometers measure an object's acceleration relative to free-fall"? Can everyone agree on that? --Steve (talk) 02:39, 23 January 2009 (UTC)

Blocked both G and W for 3h as a genlte reminder to stop edit warring. Meanwhile the edit they are warring over [2] has both sides wrong. There is no difference between inertial acc and gravitational. Ask Einstein (but don't ask Newton) William M. Connolley (talk) 08:41, 23 January 2009 (UTC)

Trying to resolve in discussion here

Wolfkeeper. The below paragraph serves as a template for the remainder of the Gravitational and inertial acceleration section. That section builds upon that paragraph. Let’s start there. Please explain precisely what is wrong with the following paragraph:

---

An accelerometer measures acceleration in one or more axis. It responds to both gravity and inertial acceleration^[1]. If you orient a stationary, single-axis accelerometer so its measuring axis is horizontal, its output will show zero gee. Yet, if you rotate the accelerometer 90° so its axis points upwards, it will read +1 g upwards even though still stationary. If you mount the accelerometer in an automobile with its axis aligned forward with the vehicle’s direction of travel, and drive down the road at a constant speed, it will read 0 g. Yet, if you hit the brakes, it will read about −0.9 g. Accelerometers respond equally to gravity and inertial acceleration.

1^ MEMSIC: ACCELEROMETER PRIMER

---

Please explain clearly and exactly what it is you dispute. Then we can go to Wikipedia:Dispute resolution if necessary. Greg L (talk) 18:32, 22 January 2009 (UTC)

Even in the bizarre world you inhabit, it should be self-evident to you that accelerometers don't respond equally to gravity in the same way as other accelerations. If I accelerate an accelerometer downwards it registers positive g downwards. If I have an acceleration due to gravity- gravity is always downwards, but the accelerometer shows positive g upwards. It therefore isn't even in your terms 'equal' anymore than -1 = 1 - they respond oppositely to gravity than other accelerations.- (User) Wolfkeeper (Talk) 20:28, 22 January 2009 (UTC)

And if I let it fall- it accelerates- downwards (at, by shear coincidence 9.81m/s^2)- giving a reading downwards, and it adds that to the acceleration due to gravity (which is unchanged)- and reading upwards; and gives a total zero reading. Right? They're not equal. One is inverted to the other. Equal and opposite.- (User) Wolfkeeper (Talk) 20:28, 22 January 2009 (UTC)

• When you begin a post with Even in the bizarre world you inhabit, you are being confrontational, are being uncivil, and make it exceedingly difficult for other editors to take you seriously. I suggest you go cool off and come back when you can be constructive and engage here in good faith. Fortunately, I’ve got a pretty thick skin and have little inclination to go run off to mommy and complain about how some such editor is calling me “a poopy head”. But if you want to enjoy the privilege of posting a {disputed} tag at the top of the article, it’s time you started getting into the saddle here and abide by the rules of Wikipedia.

  More importantly, I’ve repeatedly asked you to explain precisely what it is about the text that is in error and we are repeatedly met with explanations about how you think the world works that only leaves us guessing as to what you think is wrong with the text. So…

  Please copy the above passage, paste it below, strike what you think is inaccurate, follow it up with what you think is the correct language, explain your reasoning, and cite your basis.

  Now, so we don’t waste time on fundamentals, no accelerometer in the world can tell the difference between sitting on your desk, where it is being exposed at 1 g to the force of gravity, and sitting inside a space ship which is accelerating out in space away from earth at 1 g. To the accelerometer, they are absolutely identical and indistinguishable. Even to a light beam the two accelerations are absolutely identical. Further, if you turn a single-axis accelerometer 90° on your desk, it will read zero. And doing the same thing to the accelerometer in the space ship would have the exact same effect.

  I suggest that if what you are driving at is some sort of nuance that goes beyond this, it may well be beyond the scope of this article. Greg L (talk) 20:51, 22 January 2009 (UTC)

• What you're missing is that the accelerometer on your desk is not reading 1g because of gravity. It is reading 1g because the desk is pushing upward on it. Gravity is merely the reason why this push doesn't cause it to MOVE upward, but the accelerometer only measures the mechanical push, and the mechanical stress, and doesn't care whether this causes motion or not (and has no way of telling, all by itself). In every situation, the acclerometer in your pocket only measures the accelerational stress on your body. This is completely independent of what gravitational fields might be around-- they could be large or small or nonexistent, and it doesn't matter to the accelerometer. It doesn't see any of them. SBHarris 22:15, 22 January 2009 (UTC)

• If a large gravitational force were to suddenly appear next to someone with an accelerometer, for example a new supermassive black hole, then that would definitely register on the device. The claim that it (the device) would not see any of them (gravity fields) is incorrect.WorkingBeaver (talk) 22:56, 22 January 2009 (UTC)

• The equations of relativity don't allow for masses to "appear" or "disappear", as this would violate conservation of energy-momentum, etc. Consequently we don't know what would happen if a mass appeared out of nowhere; there aren't even any equations for it. However, if a black hole (or any other mass) were to "sneak up on you" in the conventional way, you definitely would never feel it (save for the tides).

• You wrote above "that figure will not change if the Earth is then removed, along with its gravitational field". You just described the Earth suddenly being removed, i.e. disappearing. You just contradicted yourself. A super massive black hole moving towards someone would definitely be felt by the device.WorkingBeaver (talk) 23:40, 22 January 2009 (UTC)

• You’ve come a long way, Sbharris, in understanding this. You first started off by stating that gravity is a mechanical pressure from an electromagnetic force [3] No, I’m not missing anything. What you are missing is that the accelerometer is reading 1 g because of two forces: gravity pushing down on the entire accelerometer (including its contents), and the desk pushing up on the accelerometer’s body.

  I’m thinking I should add that wording to the article. Greg L (talk) 22:29, 22 January 2009 (UTC)

• Stop misquoting me! I never said gravity was due to pressure from an electromagnetic force. I said what you THINK you're feeling as "gravity" is pressure from an electromagnetic force. Your gut, the seat of your pants, and your accelerometer only measure acclerations that produce mechanical stress, and the stress is what they measure. An object falling in a g field has both grav acceleration, AND inertial acceleration. But there is no stress on it, and thus an accelerometer carried along will read ZERO. It does not measure sums of accelerations, it (effectively) only measures differences between them. And those differences can be zero, even though the accelerations are still there. SBHarris 23:25, 22 January 2009 (UTC)

• I see. So you just wrote what you THINK you're feeling as "gravity" is pressure from an electromagnetic force. If you still embrace that view, then one can only conclude that you haven’t come a long way; for gravity is not a pressure nor is electromagnetism involved in any shape, form, or fashion. Your recent edit made no sense. The MEMSIC citation is authoritative and clear. Wikipedia goes with reliable sources and MEMSIC is a wold-wide manufacturer of devices for measuring g-force. They state, in a primer on accelerometers as follows: Accelerometers are used to convert an acceleration from gravity or motion into an electrical signal. As Einstein himself wrote, gravity is an acceleration that nothing, not even light, (and certainly not accelerometers) is immune to. The MEMSIC primer doesn’t get into Einstein and spacetime. It simply says accelerometers measure gravity and motion. It was written so absolutely anyone could understand the concept. This is not complex. Greg L (talk) 00:04, 23 January 2009 (UTC)

Here's the table of accelerations:

gravity (downwards)	acceleration relative to Earth (downwards)	accelerometer (downwards)
1g	0	-1g (i.e. upwards)
0g	1g	1g
1g	1g	0g (freefall)
0.5g	0g	-0.5g (i.e. upwards)

You tell me Greg_L self-styled physics guru, self-styled expert article writer, why is the sense opposite in the above table?- (User) Wolfkeeper (Talk) 22:24, 22 January 2009 (UTC)

• Please answer my question. No one here can read your mind. Greg L (talk) 22:29, 22 January 2009 (UTC)

The first two sentences are half-truths that are basically wrong. The last sentence is completely wrong, as the table shows. The rest of it is OK, and on the upside is enough for a really alert or careful reader to realise you're talking complete garbage overall, and would alert them to the need to go somewhere where somebody actually took the time and effort to get it right.- (User) Wolfkeeper (Talk) 22:48, 22 January 2009 (UTC)

• As I wrote above, please copy the above passage, paste it below, strike what you think is inaccurate, follow it up with what you think is the correct language, explain your reasoning, and cite your basis. Greg L (talk) 23:02, 22 January 2009 (UTC)

• User Wolfkeeper, it's time to put an end to this, as Greg has gone into "you can't make me understand" mode, where his responses are more and more mechanical. You have the support of all physicists here, as well as the people on Wikiproject physics. The only person holding out (as usual) is Greg L. Time, then to simply outvote him. I've made some changes to the article which you may or may not like, but it occurs to me that we're only interested in "g-force" when we feel it AS a force (ie, a mechanical force). Nobody gives a damn about g-force when you're falling into a black hole, so it's not just another name for accleration. In fact, it's the measure of the units (and the common name for) the kind of accleration you can feel in your gut, and in your butt, and (just coiincidentally) the kind that is the only kind that is measured by accelerometers. So let's just note that, and move on. What do you say? SBHarris 00:03, 23 January 2009 (UTC)

Yes, I'm just trying to think of a good phrase, probably starting the article with something along those lines, something like:

G-force is a measure of apparent acceleration, expressed in terms of multiples of one standard gravity.

That's a simple and fairly useful and unambiguous definition of what we're talking about, and then we can explain how it works in more detail in the article, including the (really rather very counter-intuitive) bits about how gravity interacts with accelerometers.- (User) Wolfkeeper (Talk) 03:25, 23 January 2009 (UTC)

The rest of the lead needs to be a summary of the article in accordance with WP:LEAD.- (User) Wolfkeeper (Talk) 03:25, 23 January 2009 (UTC)

• A vote on a point of view that is wrong still would not pass and make it to the article so don't try to force changes into the article that would damage it. Instead try to actually answer Greg's questions on the subject because at the moment with you jumping up and down shouting doesn't provide a good argument. WorkingBeaver (talk) 01:09, 23 January 2009 (UTC)

Rate of change in acceleration

We’ve now got an article on g-force that explains acceleration in more familiar, accessible terms than one often finds at NASA and other scientific sources. It now finally explains a very important aspect of g-force: the role of *force* in accelerations. Next, it is probably time for a short intro to Jerk (physics). That article includes this sort of stuff:

${\vec {j}}={\frac {\mathrm {d} {\vec {a}}}{\mathrm {d} t}}={\frac {\mathrm {d} ^{2}{\vec {v}}}{\mathrm {d} t^{2}}}={\frac {\mathrm {d} ^{3}{\vec {r}}}{\mathrm {d} t^{3}}}$

…which isn’t highly accessible to a wide audience. I would propose to add a short section on jerk. This article, g-force, is where many readers would first go to try to learn of such a thing since hardly anyone would know to type “jerk” or “Jerk (physics)” into our search field. Here, we can briefly introduce the concept, explain it in terms that make it extremely easy to conceptualize (I have a couple of ideas), and then link to the main article. Greg L (talk) 19:47, 22 January 2009 (UTC)

I would prefer that type of discussion be in the Shock (mechanics) article. Jerk only applies to transient shocks and not sustained g-forces. The shock article could use a little input on jerk and on dynamic response to short term shock. Rlsheehan (talk) 22:11, 22 January 2009 (UTC)

• Along with touching upon the subject of sinusoidal accelerations (shaker tables, for instance) to ensure readers don’t walk away from here with the notion that g-force is only a long-term phenomenon, John had also suggested we touch upon jerk [4]. I agree with him on both counts; briefly touching upon jerk here and linking to the main article would serve a valuable end. Greg L (talk) 22:19, 22 January 2009 (UTC)

SBHarris and Wolfkeeper

SBHarris and Wolfkeeper I am going to have to ask that both of you stop editing the article and only continue to talk on this page and that you both calm down. This is because it has become apparent from reading your contributions that you do not fully understand the subject and that your increasingly heated edits are damaging the accuracy of the article, so I think it is better that both of you try to talk here to understand the subject better before trying to contribute to the article page. WorkingBeaver (talk) 23:49, 22 January 2009 (UTC)

You're going to have to ask us?? And just who are you, Gameboi? I see you've been on Wikipedia 5 months and during that time, have a grand total of 62 edits to your name, mostly on Super Nintendo Games (!) I would politely suggest you find somebody who knows what they are doing on Wikipedia (based on your experience, unless you're a sock, that would not be yourself), and meanwhile, feel free to demonstrate to us your knowledge of physics. I have a better suggestion, even: stay out of this. If there's anybody here who is out of their league, that would appear to be you. But feel free to convince me otherwise. SBHarris 00:27, 23 January 2009 (UTC)

No, I agree with WorkingBeaver, he would have to ask us that (LOL), I find that people do tend to beg me to stop as I systematically demolish their arguments with unassailable facts. :) - (User) Wolfkeeper (Talk) 03:53, 23 January 2009 (UTC)

I take the fact that you use personal attacks to indicate that you don't know what you are writing about which then goes to demonstrate my point about how you need to calm down and how your heated edits are damaging the article.WorkingBeaver (talk) 01:01, 23 January 2009 (UTC)

You have next to no experience at Wikipedia. You have yet to demonstrate any knowledge of physics at all. These are not personal attacks, but rather relevent facts to this issue. SBHarris 01:31, 23 January 2009 (UTC)

You are making personal attacks by the way "who the *&^% are you, Gameboi?" (swearing and name calling) "have a grand total of 62 edits to your name, mostly on Super Nintendo Games (!) LOL." (ridicule) "I have a better suggestion, even: stay out of this." (threatening language) "If there's anybody here who is out of their league, that would be you." (obvious personal attack when combined with the previous quote). I have enough experience to see that you are continuing to use personal attacks instead of making a strong argument for your point of view, this is because I don't need any experience on Wikipedia to see when someone is making a weak argument. WorkingBeaver (talk) 01:55, 23 January 2009 (UTC)

The other options are to protect the article or to block the editors. It's your choice. --Carnildo (talk) 01:16, 23 January 2009 (UTC)

OK, I can bring 8000 edits to the table and I'll still ask everyone to calm the hell down. S and W, that includes you. Greg, I'd really advise you to think this over, you're not talking to block-heads here. You seem to be very much relying on the statements of a MEMS manufacturer, which are not reflective of the world of physics as much as they try to provide a simple explanation for the potential purchaser, rather than a deep understanding. In fact, it looks recently like you're providing behavioural evidence. Please everyone, step back a bit and think about exactly which part of the encyclopedia you're actually trying to write here. Franamax (talk) 00:55, 23 January 2009 (UTC)

• Fran, I know you're trying to do the right thing, which is why I'm going to point out that my 12,000 edits and several more years here, give me longer a WP:DICK than yours, too. :) Now, please consider that people making edits are going to turn into editors if somebody comes in and says something they all know to be wrong. I'm shortly going to go over to Wikiproject Physics and get some of those physics prof guys to come here and say that, but the process is not something anybody needs to be ashamed of. The same would happen if some guy turned up in the WW II article and started reverting everyone who said the Germans didn't win it. That's not editwarring. It's merely getting rid of a nutty point of view. Poor Wolfkeeper up to now has been trying to do this alone, but he's getting help as the physics community realises what's going on, here. Eventually, GregL's wrong understanding of how accelerometers work, and what they do, will be erased. Perhaps too much emotion will go into it. But there's no help for that. Not every argument has two sides, and shades of gray. Sometimes, people are simply wrong. SBHarris 02:09, 23 January 2009 (UTC)

• I never said editors were block-heads. I do find that SBHarris’ claim that gravity is an “electromagnetic force” [5], [6] to be very illuminating about his understanding of the subject.

  The central issue should be the text in the article I’ve repeatedly asked Wolfkeeper to explain what he thinks is inaccurate. I’ve asked him to please copy the above passage, paste a new version of it, strike what he thinks is inaccurate, follow it up with what he thinks is the correct language, explain his reasoning, and cite his basis. That is not too much to ask but all we get is run-around.

  What MEMSIC says about accelerometers measuring both motion-based accelerations and gravity—Accelerometers are used to convert an acceleration from gravity or motion into an electrical signal)—is in perfect accordance with real-world physics and for anyone to state that MEMSIC—a world-wide manufacturer of accelerometers for industry and the military is wrong and you, Franamax, are right because you have 8000 Wikipedia edits doesn’t pass any reasonable *grin test* here.

  There seems to be a great deal of willingness to ignore authoritative sources, including an above-mentioned book, which said [An accelerometer] cannot distinguish between inertial acceleration [a-bar] and gravitational acceleration. This view (from a book on the design of inertial guidance systems) is in accordance with what Georgewilliamherbert wrote above: In clear terms: There is no difference between a gravitational and an inertial acceleration, per Einstein. We're sitting in a 1-G static gravitational field. He was brought in by Wolfkeeper as an expert. Let’s ignore him too.

  The issue seems to be (it’s anyone’s guess until Wolfkeeper et al. respond with specifics) over the article saying that accelerometers respond to gravity just as well as to inertial accelerations. That is blindingly obviously and completely true. Greg L (talk)

• It is true by the equivalence principle that "inertial forces" aka ficticious forces (like centrifugal force) cannot be told apart from gravitational forces. But alas for Greg L, that's no help, because accelerometers not only don't feel gravitational forces, they don't feel fictious or "inertial forces" either. They are blind to all of these. The ONE AND only thing that makes an accelerometer register, is to poke it or push it, and that with a conventional mechanical force (like a push from the hand or ground; something that would destroy it if intensified, because it is transmitted only mechanically through the material of the device from the point from which it is being pushed). That is all. And it's very simple, too. SBHarris 03:40, 23 January 2009 (UTC)

• The claim "The ONE AND only thing that makes an accelerometer register, is to poke it or push it, and that with a conventional mechanical force (like a push from the hand or ground" is completely false. Think about a spring mass accelerometer floating in free space and a very large mass unconnected and a very long distance away from the device which then approaches at a large speed very close but doesn't touch the device and proceeds to pass by the device. The mass in the device will be attracted by the gravitational attraction to the very large mass obviously with varying strength due to inverse square law. This is turn will cause the accelerometer to register varying acceleration caused by the very large mass. The device of course is free floating so nobody is pushing it or holding it. The accelerometer therefore measures that which is directly linked to the effect of the changing gravity at the device caused by the large mass. Obviously this refutes the "one and only" claim made above. WorkingBeaver (talk) 04:05, 23 January 2009 (UTC)

====

*QUOTE (GaryL):

I do find that SBHarris’ claim that gravity is an “electromagnetic force” [7], [8] to be very illuminating about his understanding of the subject.

COMMENT: Since neither of the cites (only one of which is my writing) have me stating that gravity is an electromagnetic force, and since I've pointed that fact out to you already above, I find it very illuminating that GaryL continues to misrepresent me. Doesn't he think anybody will click on the links? Anyway, I'm not getting though to HIM, no matter how I say it. But I'll try for those others reading.

In the limit of a divergency free-field (small or zero tides) You cannot feel the force of gravity, for it does not causes any mechanical stresses, which is ALL that your body senses or is capable of sensing. Since such mechanical stresses are also what an accelerometer senses, it should be clear that an accelerometer is merely a mechanical version of your body, insofar as what it can do. It cannot sense gravitational fields; all it senses is mechanical strain, like a fancy spring-scale. Now, can you USE an accelerometer to measure gravity? Yes, in a limited indirect way, and VERY special circumstances where you do a bunch of things to make SURE that all mechanical strain the device sees, is caused by resistance to the gravity field (like make sure the thing is not moving in any way). And in that case, the device outputs the acceleration of gravity (or seems to), but tellingly with the wrong sign, because it's really measuring the mechanical support force that opposes the gravity, not the acceleration of gravity. And all this isn't very interesting anyway, because most of the uses for accelerometers have them moving and accelerating, and in those cases, all bets are off for using the device even to measure gravitational acceleration indirectly. So that must be added by hand, and the manufacturers all admit this. In any case, it's always wrong to say that accelerometers respond to gravity. They don't, period. They respond to outside mechanical pushes. Which are all non-gravitational forces. SBHarris 02:02, 23 January 2009 (UTC)

Since Sbharris claims "Since neither of the cites have me stating that gravity is an electromagnetic force" and yet the cite given by Greg [9] shows "All you feel is mechanical pressure from an electromagnetic force." then Greg has shown that Sbharris's claim is false. WorkingBeaver (talk) 03:35, 23 January 2009 (UTC)

Not if what you feel isn't gravity. Which, of course, it isn't. You can't feel gravity. If all that acts on you is gravity, you feel nothing. No g-force (interestingly). SBHarris 21:20, 23 January 2009 (UTC)

To clarify: my 8000 edits qualify me for nothing more than to ask you to all calm down. They make me no more right or wrong on the subject matter. They do indicate that I've been around long enough to spot when the comments stray over the line from edit to editor. That seems to be the case here. Franamax (talk) 01:38, 23 January 2009 (UTC)

• Very well. Thanks for your intervention, Franamax. Wikipedia follows most-reliable sources. MEMSIC is clearly very authoritative and their document, ACCELEROMETER PRIMER is profoundly clear. The document is clearly intended to start with the ultra-fundamentals. This MEMSIC citation has the additional virtue of asking that we not ignore Einstein’s Theory of General Relativity, which states that gravity and inertial accelerations are identical and indistinguishable from other. Any accelerometer than can sense inertial acceleration must also respond to gravity, otherwise they wouldn’t work at all. Greg L (talk) 01:42, 23 January 2009 (UTC)

Sbharris - you seem to be under a bit of a misconception when you state "the wrong sign". A freely falling system experiences the cancellation of gravitational by inertial forces (another restatement of the principle of equivalence, e.g. Weinberg, Gravitation and Cosmology, p. 68). In other words, the Earth's gravity (from the observer point of view) is equivalent to an acceleration upward, not downward (as a trip in an elevator should convince you). Hal peridol (talk) 02:20, 23 January 2009 (UTC)

The Earth's gravity is equivalent to a trip in an elevator upward (presuming the elevator is in space, and drawn up on a rope by Einstein's genie). The problem is that the accelerometer does not see any "inertial forces" from this, either. Start with an elevator in space, not moving. The accelerometer floats in the center of the elevator, registering zero force, until somebody pulls the elevator and the floor runs (smack) into the accelerometer. At which point, a mechanical force acts on the device, from the floor. That's basically the end of the story, and the equivalence principle never has a chance to appear. The acclerometer, wired to report when it's being pushed by a mechanical force, does what it's supposed to and reports that force, and the acceleration associated with it, and the direction which it's coming from. Which is out of the floor. Okay? Mystery solved. And when you let the device drop in the middle of a nonmoving elevator on Earth, the same thing happens. It fetches up against the elevator floor, and reports that the floor is pushing it upward, mechanically. Also, end of story. In neither case are inertial forces or gravitational forces seen by the accelerometer. It sees the floor and the push from the floor. SBHarris 03:58, 23 January 2009 (UTC)

Unfortunately, under the wikipedia rules it's not very authoritative at all. The verifiability guidelines (Wikipedia:Verifiable#Reliable_sources) state:

Articles should rely on reliable, third-party published sources with a reputation for fact-checking and accuracy.[4] Reliable sources are necessary both to substantiate material within articles and to give credit to authors and publishers in order to avoid plagiarism and copyright violations. Sources should directly support the information as it is presented in an article and should be appropriate to the claims made: exceptional claims require high-quality sources. In general, the most reliable sources are peer-reviewed journals and books published in university presses; university-level textbooks; magazines, journals, and books published by respected publishing houses; and mainstream newspapers. As a rule of thumb, the greater the degree of scrutiny involved in checking facts, analyzing legal issues, and scrutinizing the evidence and arguments of a particular work, the more reliable it is. Academic and peer-reviewed publications are highly valued and usually the most reliable sources in areas where they are available, such as history, medicine and science. Material from reliable non-academic sources may also be used in these areas, particularly if they are respected mainstream publications. The appropriateness of any source always depends on the context. Where there is disagreement between sources, their views should be clearly attributed in the text.

This is just advertising copy that Greg_L is referring to here, which has very probably been hacked together by some guy in that company and may well not have had any fact checking or scrutiny at all on it. Also, it's not in their interests to go into the full gory details; and it's literally a cartoon like oversimplification of what these devices do for you, and doesn't cover how they work.- (User) Wolfkeeper (Talk) 03:53, 23 January 2009 (UTC)

Steve's proposal for the introduction

My view is that every argument on this page is moot, because any phrasing that isn't obviously and straightforwardly correct to every single editor here is a phrasing that's not clear enough for the article. It doesn't matter whether it's technically correct or not. So why spend time arguing?

In that spirit, here's my proposal for the introduction:

A g-force is a measurement of the total effective "push" that one feels due to the combined effects of gravity and acceleration. (Despite the name, it is not precisely a force in the physics sense.) It is conventionally measured in units of “gee” (symbol: g), pronounced /ˈdʒiː/. The g-force is defined to be "1 g" for a stationary object subject to gravity on the earth's surface; it would be "0 g" in any "weightless" environment such as free-fall or an orbiting satellite; and it would be greater than "1 g" in a rapidly-accelerating car, for example.

To be precise, if a is an object's acceleration vector (in an inertial reference frame), and g is the gravitational acceleration vector (so that the gravitational force on an object of mass m is mg), then the g-force vector of the object is (g-a), while the magnitude of the g-force, in units of g, is |g—a|/(9.80665 m/s²). (The denominator is standard gravity.)

The gee and its symbol, g (or G) should not be confused with the universal gravitational constant, (symbol G), which is a physical constant that fundamentally relates mass and gravitational attraction.

Thoughts? For example, is there any respect in which this description is not obviously and straightforwardly correct? If so, let's discuss ways to improve it. :-) --Steve (talk) 03:07, 23 January 2009 (UTC)

Don't mention gravity in the first sentence, just say it's the total apparent acceleration divided by standard gravity. Hopefully we can all get behind that POV (and it is, I would, say NPOV).- (User) Wolfkeeper (Talk) 03:53, 23 January 2009 (UTC)

• Agree. Mentioning gravity just makes the sentence wrong. G-forces by popular consent are things that are felt, and which (if too intense) will squash or kill you. Neglecting gravitaional tides due to field divergence (which produce tidal forces, not g-forces) nobody ever got squished by a purely gravitational force. Even as you fell through the 1000-g zone of a black hole, and as a result accelerated at 1000 g's, you'd feel no g-force. In fact, gravity doesn't contribute directly to ANY "g-force" (what this article discusses). Again: gravity dose not contribute to the sensation that the phrase "g-force" is used to describe, as the subject of this article.

  Resisting gravity surely does cause "g-force", but then you're feeling the force from THAT resistance, not from the gravitational force itself. The gravitational-force, like all inertial/ficticious forces, is gentle on the soul and kind to the skin, and soothing to the complexion! Basicially, it's not an issue for beefy astronauts or jet pilots to bulk up to fight. There's nothing to fight. Gravitational forces not only do not produce g-forces, but they can't even be felt at all! So stop worrying about them producing g-forces that make your face sag or that will drain the blood from your brain, is my advice.

  Next, we have the idea that these nasty g-forces are caused by acceleration. How awful. But again, it's not strictly true. That's putting the cart before the horse, and confusing the effect for the cause. The force causes the acceleration, not the other way around which is implied by saying that acceleration causes g-force. And when the accelerating force is the bad, nasty mechanical kind, that sort of force is what produces g-force! In fact, it's the only kind that does!

  You can tell for certain when the accelerating force is this bad, nasty, deforming mechanical kind, because something uncomfortable is poking you in some tender place, and all your organs are being pulled at, by other organs and ligaments and such, which all connect mechanically and materialistically back to that tender place. Thus, g-forces are caused by something pushing you or pulling you in a very mechanical way, period. Look closely and you can always see what it is! It looks like the floor or a seat or Or some collection of belts, or (if you forgot the belt, like Pricess Di) maybe the windshield or back of the front seat, as you impact it. During ia plane crash, when the plane stops and you, the passenger, don't, the g-forces that finally kill you are always produced by something that stops you from your comfortable forward flight, but meanwhile, you're fine. But what g-forces NEVER are, is some kind of invisible field like that in a B-grade monster movie, that reaches into you and produces bad effects. G-forces are produced by OBJECTS OUTIDE OF YOU. And those OBJECTS are not invisible: rather, they are poking you or pushing you or prodding you. And in NO OTHER WAY are these g-forces produced, at least on the human body (for accelerometers, they are produced by poking on the accelometer, and that's the only way you can get it to respond).

  I sometimes think we might all have it better if we'd agreed to write this article up for Simple Wiki. Okay, somebody is going to say: don't you have g-forces produced by the acceleration of (say) a rocket? And the answer is: no. If you're not attached to said rocket, it will go up and right by you, and (neglecting the exhaust heat) you can wave and won't feel a thing. No g-forces. "But wait," somebody objects. "You are attached. You have a seat under you, and belts and stuff." Well, there you are! The seat and the belts produce the g-forces on you. Without them, the rocket or the centrifuge or the auto would be powerless to make you feel any forces at all. To be g-forced, you have to be pushed, poked, prodded. No fancy fields. No silly fictious accelerations, or whatever. Rather, it takes some gross item which is physically externally jabbing, tugging, or otherwise molesting you. That's it. The lead of the article should say this. SBHarris 04:30, 23 January 2009 (UTC)

• Again it is wrong to claim "If you're not attached to said rocket, it will go up and right by you, and (neglecting the exhaust heat) you can wave and won't feel a thing. No g-forces." because it ignores the force between two objects that have any mass. If the rocket is suitably massive enough, like planet sized, then the effect of the rocket/planet moving by you is easily detectable. Just because the rocket is a relatively small mass compared to a planet it doesn't mean you can make sweaping statements that ignore those effects. g = Gm1m2/r^2, understand the consequences of that equation when talking about spring-mass accelerometers and objects with suitably large mass. WorkingBeaver (talk) 04:58, 23 January 2009 (UTC)

• Entirely missing the point of the thought experiment by focusing on low-low-low order effects. For heavensake, don't ever take any engineering classes. SBHarris 05:16, 23 January 2009 (UTC)

• The effect is detectable with current technology with objects much smaller than a planets so it is not insignificant and if you knew enough about this subject you would not have written what you did. That demonstrates to me that you don't know enough about this subject to be editing the article. WorkingBeaver (talk) 05:30, 23 January 2009 (UTC)

no, there would be gravitational waves and tidal forces but they're different.- (User) Wolfkeeper (Talk) 07:54, 23 January 2009 (UTC)

No you're forgetting that this effect is detectable with current technology and as such accelerometers can detect what Sbharris claimed was not happening. Sbharriss is completely wrong thus proving my point. WorkingBeaver (talk) 10:46, 23 January 2009 (UTC)

You're discussing the misicule gravitational pull of a rocket next to you, in a thought experiment which has nothing to do with tiny gravitational pulls. And you deliberately trying to sidetrack and confuse this discussion? If you want to do math, compare the gravitational pull of a 240,000 pound space shuttle which averages 100 feet away, to that of a 240 lb person standing next to you, who averages 1 feet away. What is your point? Do yo have a point? SBHarris 11:05, 23 January 2009 (UTC)

• No SBHarris. That is POV-pushing from someone who doesn’t want to accept the obvious. Drop this anti-gravity bent; the evidence is clear that gravity is an acceleration and accelerometers can’t distinguish between 1 g of gravity and 1 g of inertial acceleration. We follow reliable sources on Wikipedia.Greg L (talk) 04:37, 23 January 2009 (UTC)

Do you even know what an "inertial acceleration" is? It's a ficticious acceleration/force in an accelerated frame, like centrifugal force. I suppose you do remember all the time you wasted for all of us on the centrigual force wiki, trying to convince us that centrifugal force could be somehow felt. But please read what I wrote above, as regards accelerometers. Yes, accelerometers can't distinguish between gravitational and inertial forces. But that's because they can't feel EITHER of them! An external force applied to an object to make it move, is not an "inertial force." It's a mechanical force. Accelerometers CAN see that kind of force, but that kind of force is easily told apart from gravity.

• I think it would be confusing and unnecessary to say in the intro that g-force is not technically a force. That sort of caveat can wait. Clearly, 1 g is simply an acceleration. However, setting aside Einstein’s views of warped spacetime and focusing on the Newtonian view of things (F = ma), one gee due to gravity is purely the product of an applied force, which generates (for stationary objects), a countering force. Further, all inertial accelerations are the result of imbalanced forces. I strongly suggest we avoid the temptation to try to treat the first introduction as a synopsis of the entire article (with caveats about what it technically isn’t when it has already been clearly stated what it quite technically is). The intro, IMO, should limit itself to fact that the measure of g-force is the measure of accelerations, provide the units, symbols, and the magnitude of the SI value. I read once that one of the hallmarks of Wikipedia was its short, pithy introductory paragraphs; I see no need whatsoever to depart from this practice. Greg L (talk) 04:37, 23 January 2009 (UTC)

Trouble is Greg_L, there's different sorts of accelerations, and accelerometers can't DIRECTLY read accelerations that are due to pseudo-forces at all, like centrifugal force, coriolis force... and gravity. These forces are really aspects of momentum, and are not real forces. However if the force is for want of a better word 'frustrated' by a real, opposing force, then the accelerometer suddenly reads the acceleration due to that real force. That's what all the Einstein stuff is about with the equivalence principle, he says that gravity is equivalent to an accelerated reference frame and that makes it a pseudo-force, not a real force. Accelerometers cannot read momentum, because it isn't, truly an acceleration, and that's why gravity gives no g-force (as in freefall for example).- (User) Wolfkeeper (Talk) 05:21, 23 January 2009 (UTC)

Look, I didn't make up the idea for this ill-conceived article. So far as I can tell, this article was meant to be about the forces (usually measured in g's) which one feels due to the application of mechanical forces to objects or people. So let's let it be about that, and that only. Put that in the LEAD! Neither gravity nor the fictious forces that are sometimes called "inertial forces" (in accelerated frames) produce by themselves and without any other help, any stress in objects. And thus their effects are not measured in g's, in part because they cause no problems. Only mechanical forces causes problems and stresses (always neglecting tides). So let us let this article talk about accelerations induced by purely mechanical forces, and take the rest of the crap out of it, as being irrelevant. And confusing. SBHarris 05:13, 23 January 2009 (UTC)

SBHarris and Wolfkeeper, you understand that the first sentence is somewhat colloquial, right? People have an intuitive understanding that the force on their butt when they sit down is the force of gravity. I know what you're thinking: These people and their intuitive understanding are wrong! Their butts are actually feeling the upward force of the chair, not the downward force of gravity! And you are perfectly correct. But you can't deny that most readers think this way (wrong though it may be), and we shouldn't be happy with an introduction that won't make sense to those readers.

So how about: "A g-force is a measurement of the total effective "push" that one feels, associated with gravity and acceleration." It's a little vaguer. Now, it's not saying that gravity is the direct cause of a g-force, it's only saying that gravity can have something to do with g-forces: For example, gravity is right now setting up the situation in which my chair can apply g-forces to my butt. Is that phrasing OK? :-)

Greg L, I agree that the "not actually a force" thing should be removed. --Steve (talk) 05:29, 23 January 2009 (UTC)

Well, I'm prepared to go with that first sentence. It's not actually wrong, although it is rather misleading. The current first sentence is actually, clearly, wrong. Do it, I'll back you, and please remove that god-awful link to that ghastly sales material while you're there, otherwise I will not back you; that is not a reliable source.- (User) Wolfkeeper (Talk) 07:43, 23 January 2009 (UTC)

I've put it in. The problem is, if only acceleration and not gravity is mentioned, a non-physics-y reader will say, "OK I'm not accelerating when I'm standing in place on earth, so there must not be any g-forces on me." Do you have a suggested phrasing that those readers would be able to immediately relate to? Alternatively, I could make it even vaguer, "A g-force is a measurement of the total effective "push" that one feels related to gravity and acceleration." --Steve (talk) 18:15, 23 January 2009 (UTC)

Now it's been entirely reverted by Greg L, with the comment, "That doesn’t work at all. It discusses vertical accelerations such as gravity and freefall, and then says 1 g of gravity increases in a car; only if it has rockets and is going UP. Keep it simple." OK Greg, if I replace the car example with an upward-accelerating rocket, would you then be happy with this intro?

By the way, the reason I'm not happy with the replacement "The measurement of g-force (or g-load) is the measure of an object's acceleration." is because a reader will see this and say, "If I'm standing stationary on the ground then I'm not accelerating, and therefore I must not be experiencing any g-force." This reader will start the article already confused. --Steve (talk) 21:14, 23 January 2009 (UTC)

Why not professional review

• I have an idea. Why don’t we invite a Honeywell engineer to review this article for accuracy and have them e-mail their response to a couple of editors here? I think that beats revising the article because a couple of editors here think gravity is an electromagnetic force and also think gravity has nothing to do with accelerations and g-force. Rather than invent a house style on the laws of physics, we simply invite a pro to weigh in on the subject. This, BTW, is what I did on Kilogram: I exchanged about sixty e-mails with the guy who heads the NIST’s watt balance project. He even forwarded some of my e-mails to the head “kilogram guy” in Paris for feedback. Now that I see what is going on here, I’m thinking this sort of approach (getting editorial feedback from experts) is in order here now. How say ye all? Greg L (talk) 04:46, 23 January 2009 (UTC)

Nah. A professional physicist would be what you want.- (User) Wolfkeeper (Talk) 05:08, 23 January 2009 (UTC)

• I see… Well, the issue (as best as I can tell since you have refused to be specific about exactly what wording your disagree with) is whether or not accelerometers respond to gravity in the same way as they respond to inertial acceleration. I should think a degreed engineer working for a world-wide company that designs and manufactures accelerometers would be more than sufficient. It certainly would meet two important criteria: WP:Neutral point of view, and WP:Reliable sources. And it certainly beats what you might *possibility* have in mind: cherry picking a Wikipedia editor, where it is exceedingly difficult to vet their experience.

  I’ll make some calls tomorrow. Anyone who wants to receive the engineer’s response can e-mail their e-mail address to me here. For those that don’t care to directly receive the engineer’s response, I will post it here for the benefit of all. Greg L (talk) 05:30, 23 January 2009 (UTC)

• I'm afraid we all had our experience with Greg L. when he found Dr. Paul Marmet, a crank known for his odd theories on relativity, and used them browbeat about half a dozen physicists on the Mass-Energy Equivalence page, until he was finally forced to give it up by pure force of numbers of experts laughing at him. He'll do that if you let him. I suggest you peruse this, as light reading, if you intend to get into a physics war with Greg L and his private experts. Enjoy:

http://en.wikipedia.org/wiki/Talk:Mass%E2%80%93energy_equivalence#Matter_losing_mass_as_it_falls_down_a_gravity_well

SBHarris 05:46, 23 January 2009 (UTC)

• You still think gravity is an electromagnetic force. I don’t think anyone here is interested in the fact that SBHarris has certain views on special relativity.  ;·) Wikipedia requires reliable sources and neutral point of view. We can not cite Wikipedia editors. And we certainly won’t be revising our Gravitation article to say that it is an electromagnetic force and then add a citation that says “Cause SBHarris says so”. Greg L (talk) 05:58, 23 January 2009 (UTC)

Don't tell me what I think, Greg. SBHarris 06:04, 23 January 2009 (UTC)

He never said that Greg_L; please quote the entire paragraph where you claim he said that or STFU (and I want the entire paragraph, because I've seen you quote out of context multiple times.)- (User) Wolfkeeper (Talk) 06:16, 23 January 2009 (UTC)

• I wouldn’t tell you what you think. No need. You told us what you think. Twice. Gravity is an “electromagnetic force” that creates a “pressure”.[10], [11]. That is false and absurd. For one thing, it isn’t electromagnetic. Not in the least. For another pressure is force per unit area. Gravity is just an acceleration.

• It's amazing that we have to quote something here which is freely available, a click away. I wrote in the first link you give above: You're not feeling gravity, either. All you feel is mechanical pressure from an electromagnetic force. Food stays on the botton of your stomach because your stomach pushes up on it. That's pretty simple physics. Food in your stomach doesn't fall. It would fall if your stomach wasn't pushing up on it. That push is between the atoms in your stomach, and the atoms in the food. This force is electromagnetic in nature (there are four classical forces, and this force is not gravity-- so which do YOU chose, Greg L?). Accoding to Einstein, this force is the only one that exists in the picture, so you surely know that I'm NOT talking about some other force (like gravity). If you want 2 forces to be classical, there's at least one other force besides gravity in the picture, and this is it. For Einstein, this atom-to-atom force is the ONLY force. In the Newtonian view, gravity is a force pushing the food downward, but whichever view you take, you can't feel the force that makes the food tend to go downward. You can't feel it either because it doesn't exist (Einstein's view) or because your stomach is supported against falling by your body, which is in turn supported against falling by the floor, and all you can tell with your nerves is that there's some mechanical stress involved with keeping things from falling, using your body. You feel mechanical pressure, not gravity. Even Newton's view doesn't suggest you can feel gravity. All you feel is the tissue-pressure associated with keeping things up, when they want to fall downward. You feel the mechanical stress from weight of the food, but in a rocket you'd feel the same mechanical stress from inertia of the food. Mechanical electromagnetic stress-strain is the only relevant sense you have.

  In the second quote I say: Stop misquoting me! I never said gravity was due to pressure from an electromagnetic force. I said what you THINK you're feeling as "gravity" is pressure from an electromagnetic force. Your gut, the seat of your pants, and your accelerometer only measure acclerations that produce mechanical stress, and the stress is what they measure. Also clear. Nothing measures gravity directly-- certainly not in your body. What's available to your body and to instruments is measurement of pressure, stress, motion, force. Gravity can be inferred (when you have kept track of where your instrument is), but othertimes nothing can be inferred from what the instrument reads. The same goes for your body sensors, which use one of the four fundamental forces, but gravity isn't it.SBHarris 08:24, 23 January 2009 (UTC)

• I will remind all that this started with me and other editors correcting utterly fallacious claims, such as the Canadian government manual of style says that the proper symbol is italic g. It turns out that was a total fabrication. Next, was an utterly preposterous and absurd assertion that accelerometers can only respond to inertial accelerations and not to gravity. Patently false. The MEMSIC citation is good enough for Wikipedia. It states “Accelerometers are used to convert an acceleration from gravity or motion into an electrical signal.” That puts that issue to bed. The purpose of having an engineer who designs accelerometers weigh in on this is to ensure all the rest of the article is correct. Even though SBHarris and Wolfkeeper might not agree with the engineer’s views, Wikipedia has no room for editors who refuse to get the point. That policy states as follows:

In some cases, editors have perpetuated disputes by sticking to an allegation or viewpoint long after it has been discredited, repeating it almost without end, and refusing to acknowledge others' input or their own error. Often such editors are continuing to base future attacks and disruptive editing upon the erroneous statement to make a point.

Having editors here with their own *special* brand and understanding of physics doesn’t cut it. That’s why we rely upon reliable sources, such as the very manufacturers of accelerometers. You both should heed the advise from WorkingBeaver:

SBHarris and Wolfkeeper I am going to have to ask that both of you stop editing the article and only continue to talk on this page and that you both calm down. This is because it has become apparent from reading your contributions that you do not fully understand the subject and that your increasingly heated edits are damaging the accuracy of the article, so I think it is better that both of you try to talk here to understand the subject better before trying to contribute to the article page.

Though you may not like it, you are all spun up on making this a personal issue. It isn’t. It’s about simple facts. Greg L (talk) 06:29, 23 January 2009 (UTC)

Which bit of "please quote the entire paragraph where you claim he said that or STFU" didn't you understand exactly? I will not put up with this constant and systematic slander, lies and deliberate misinterpretation of our edits as well as the replacement of reliable sources with unreliable sales material. Your behaviour on this article is astoundingly bad and now I find out you've pulled this garbage before, and you're trying to do it again. No. Over my dead body.- (User) Wolfkeeper (Talk) 06:16, 23 January 2009 (UTC)

How inertial guidance systems work (a better paper)

Is here: http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-696.pdf

Many configurations of inertial guidance systems are discussed, but in general they use 3 devices that are gyros of one sort or another, or which correspond to gyroscopes (such as the small MEMS devices, which actually measure angular acceleration using the Coriolis effect with tiny silicon vibrating wieghts). Add to this, 3 accelerometers, one for each orthogonal axis.

The box diagrams for all of them are of interest and should be examined. Uniformly, the gyro ouputs are first used to project the measured accelerations onto 3 axes, THEN gravitational acceleration is subtracted out of the Z component, then everything is double-integrated to get velocity and finally displacement (with, of course, need to add in absolute velocity and position at each point in the integration to get rid of the constants-of-integration and give definite results).

Interestingly, one of the worst causes of error is gyros which don't get the axes precisely right, causing a small component of the subtracted g-acceleration to be subtracted not from the vertical Z-axis (as is proper), but from the X-Y plane, which is where most of the motion of interest is taking place. Since linear accelerations are integrated TWICE to get distance, small errors in integrating incorrect accelerations in the X-Y plane grow rapidly (quadratically with time) and it doesn't take too long for this bias to screw everything up. Here's the direct quote:

[section] 6.2.3 Propagation of Errors

Errors which arise in the accelerometers propagate through the double integration, as described in Section 4.2. This is the obvious cause of drift in the tracked position. Errors in the angular velocity signals also cause drift in the calculated position, since the rotation matrix C obtained from the attitude algorithm is used to project the acceleration signals into global coordinates. An error in orientation causes an incorrect projection of the acceleration signals onto the global axes. This causes several problems. Firstly, the accelerations of the device are integrated in the wrong direction. Secondly, acceleration due to gravity can no longer be correctly removed.

In the strapdown algorithm 1g is subtracted from the (globally) vertical acceleration signal to remove acceleration due to gravity before the signal is integrated. A tilt error θ will cause a component of the acceleration due to gravity with magnitude g ·sin(θ) to be projected onto the horizontal axes. This causes a residual bias due to gravity with magnitude g · sin(θ) to remain in the globally horizontal acceleration signals. There will also be a residual bias of magnitude g · (1 − cos(θ)) in globally vertical axis, however this is much less severe since for small θ we have cos(θ) → 1 and sin(θ) → 0. Hence the error in position caused by a small tilt error will occur mainly in the global xy-plane.

The propagation of gyroscope errors through to the calculated position is the critical error path in nearly all INS [Inertial Navigation system} systems. In most applications the magnitude of g is much greater than the mean absolute acceleration of the IMU itself. In such cases the critical problem is that a component of the acceleration due to gravity is projected onto the globally horizontal axes. As a concrete example consider a tilt error of just 0.05 [degrees]. This error will cause a component of the acceleration due to gravity with magnitude 0.0086 m/s2 to be projected onto the horizontal axes. This residual bias causes an error in the horizontal position which grows quadratically to 7.7 m after only 30 seconds.

Now, I hope I'm not the only one to notice that this perfectly backs up the other academic paper which has been cited, to the effect that nothing in an inertial guidance system, including the accelerometers in it, can measure gravitational g. It must be removed from the accelerometer readings "by hand" without benefit of any instrument which measures it directly. If there existed an instrument (including the accelerometers themselves) which could do this, they simply would add them to inertial guidance systems, and this growing-bias with time which results from bad orientation and incorrect g-removal angle, would not be the major problem for inertial guidance systems that it is. But it IS a problem. We've discussed the various theoretical reasons why this is not just a difficulty with technology, but rather arrises directly from general relativistic considerations: for any g field point there's an inertial frame in which you can make g vanish. And others in which it is larger or smaller. No instrument, even in theory, can tell you for a planetary g-field which is the "real" and "correct" g, at the same time you're moving up-and-down in it! The trickiness of relativity is that when navigating over a lump of inhomogenous rock, the stuff beneith you imposes on your space-time a complex pattern of "inertial acceleration" along an axis pointing vaguely to the center of the Earth, which changes from place to place, and can't be assessed "on the fly" but only if you physically stop (distance, velocity, and acceleration-wise, with regard to the center of the Earth) to measure it indirection, as you would using a simple scale. But there's no getting away from having to do that. And of course, the scale (like the accelerometer and your butt) is not measuring the g-field directly, but the stress it's causing to the supports of a resting test mass.

Anyway, we now have TWO academic papers which make it clear that accelerometers can't be used to direcly measure g, versus some company advertising and some unbacked claims by Greg L. to the contrary. I'm sorry, but Greg L's position is "pwned" by the facts.SBHarris 10:36, 23 January 2009 (UTC)

You sre simply misreading what you cite and it does not support your point of view it also does not "pwn" Greg's position. Accelerometers do respond to gravity. The links provided, even the one you cite, proves you are wrong. WorkingBeaver (talk) 10:55, 23 January 2009 (UTC)

Show your work. SBHarris 11:06, 23 January 2009 (UTC)

No, that's what is being asked of you and since you have not produced any valid work to support your point of view then you need to keep on trying. For example you contend that mass and therefore gravity has no effect on accelerometers, since you have not proven that obviously incorrect claim then I don't see how what you write can even begin to be included in the article. WorkingBeaver (talk) 11:18, 23 January 2009 (UTC)

Look at this, I managed to find the relevant bit of the book I have a hard copy of Introduction to Space Dynamics in google books: [12] pg 188 section 6.11, just like I said up^^^- (User) Wolfkeeper (Talk) 11:52, 23 January 2009 (UTC)

To that we add Eshbach's Handbook of Engineering Fundamentals By Ovid W. Eshbach, pg 9 (actually search for nongravitational gives multiple examples in this book) [13]

Also, there's the non reliable source of this forum, if you want more persuading (and a slightly simpler explanation): www.physicsforums.com- (User) Wolfkeeper (Talk) 11:52, 23 January 2009 (UTC)

Yes the physics forums are full of this:

For popular views, it's rather difficult to get anything else for such a basic physic question. However, here's a hobby article that says it better than I can in the space: http://www.lunar.org/docs/LUNARclips/v5/v5n1/Accelerometers.html

Here's a physics forum archive which does the same: http://www.physicsforums.com/archive/index.php/t-154796.html. There are many more like it, and in this forum full of physicists, nobody chimes in to say there's error.

For yet another example: http://www.physicsforums.com/archive/index.php/t-154796.html

SBHarris 12:12, 23 January 2009 (UTC)

• Like WorkingBeaver said: You sre simply misreading what you cite and it does not support your point of view it also does not "pwn" Greg's position. Accelerometers do respond to gravity. The links provided, even the one you cite, proves you are wrong.

  Wolfkeeper brought in his own Wikipedia expert and pre-announced his intention to do so by writing: I've also called in User:Georgewilliamherbert he's an admin as well as an aerospace engineer, but I've asked him to comment in a non admin capacity.

  What was poor ol’ George’s response(?): Ahhhhhh (thud) - the sound of George's head hitting the keyboard again and again. And George went on to say In clear terms: There is no difference between a gravitational and an inertial acceleration, per Einstein. We're sitting in a 1-G static gravitational field,

  Then Wolfkeeper cites an arcane formula in Eshbach's Handbook of Engineering Fundamentals that is obviously over his head and conveniently ignores where it say says [An accelerometer] cannot distinguish between inertial acceleration [a-bar] and gravitational acceleration.

  I cite a world-wide manufacturer of accelerometers, MEMSIC.com. They make sensors for “consumer, automotive, medical or industrial product applications”. It is titled ACCELEROMETER PRIMER. And it begins with this: Accelerometers are used to convert an acceleration from gravity or motion into an electrical signal.

  According to Einstein’s General Relativity, the acceleration of gravity and inertial acceleration are identical and indistinguishable. So much, in fact, that even a light beam responds to them the same. Yet Wolfkeeper thinks he is smarter than Einstein. Why? What he wrote above is emblematic of his brain-block here: Sorry, that's wrong. I'm sitting at 1g but my acceleration is zero. Acceleration is m/s^2; m/s = zero.

  Then SHHarris writes that gravity is an electromagnetic force that creates a pressure. [14], [15] No, that is absurd. “Pressure” is force per unit area; gravity is an acceleration. And “electromagnetic”? Please, light beams and radio waves are electromagnetic. As far as science knows, we can’t tune our shortwave radios to the “gravity station”. Before SBHarris weighed in here with his new science, scientists and Einstein had labored under the view gravity is a warpage of spacetime.

  You guys’ grasp of elementary physics is lacking here. That would be forgivable if you didn’t ignore clear-as-bell citations and clear-as-bell statements from editors that you brought in (trying to) tell you how it really is.

  You two are out of control. You are simply refusing to get the point and think you WP:OWN this article. All the evidence is so clear, even a five-year-old can figure it out.

  Since Wolfkeeper’s logic seems to be beyond the comprehension of Einstein and others here, I attempted to get him to point to specific wording and explain what is wrong with it, show what he thinks is correct, explain his reasoning, and cite it. I politely did this twice, here and here. All I get is absolute intransigence, personal attacks, and ducking this way and that rather than try to work on wording we both find satisfactory.

  Finally I suggest, in Why not a professional review, above, that you two stop editwarring and we bring in an expert engineer from a company that designs and makes accelerometers and have this engineer review the article and weigh in here with his take on its factual accuracy. That is met with complete opposition, personal attacks, and a stated willingness to keep on editwarring.

  WorkingBeaver hit it right on the head when he wrote SBHarris and Wolfkeeper I am going to have to ask that both of you stop editing the article and only continue to talk on this page and that you both calm down. This is because it has become apparent from reading your contributions that you do not fully understand the subject and that your increasingly heated edits are damaging the accuracy of the article, so I think it is better that both of you try to talk here to understand the subject better before trying to contribute to the article page. Greg L (talk) 15:15, 23 January 2009 (UTC)

• • Thing is, if I'm owning the article, how is it that you wrote practically all of it including specifically the extremely dubious introduction and revert warred to ensure that? There's only one attempt at ownership here, and it's not us. The other thing is: reliable sources- you don't have any. These manufacturer's 'sources' you talk about probably went through no vetting at all, they're just sales brochures. They're not verifiable, and they're extremely vague. In addition, external experts? I seem to have missed the bit where the wikipedia calls people in from outside, engineers to comment on physics for example? Or even physics for physics. When has that ever happened? Never. Never. There's no policy or guideline that says that that is valid. it's just not the way the wikipedia works- we're supposed to rely on verifiable, reliable sources. And as for consensus- the consensus from the physics knowledgeable editors is that the POV that we have of these sources we've found are accurate and correct. You don't have that. You don't even have any verifiable sources. Basically, you have nothing, your argument is flawed and your position is unverifiable OR.- (User) Wolfkeeper (Talk) 16:14, 23 January 2009 (UTC)

• No, the citations are all clear but you stubbornly refuse to believe what they say. It is you two here who are pushing OR here with unverifiable theories about how gravity is electromagnetic and other utter nonsense. Wikipedia has no room for editors who refuse to get the point. That policy states as follows:

  In some cases, editors have perpetuated disputes by sticking to an allegation or viewpoint long after it has been discredited, repeating it almost without end, and refusing to acknowledge others' input or their own error. Often such editors are continuing to base future attacks and disruptive editing upon the erroneous statement to make a point.

See the below thread. It is clear that it is utterly hopeless trying to rationally discuss things with you. I’ve repeatedly asked that you explain precisely what is wrong in the article and you repeatedly just spout theories that leaves everyone guessing as to the precise nature of what it is in the article that you believe isn’t correct. That state of affairs has all the hallmarks of someone who has been corralled into an intellectual corner. We will have an outside expert review what is in the article. It won’t matter if you attempt to declare that the outside engineer from a manufacturer of accelerometers isn’t a reliable source. It passes no ones *grin test* here that you are a reliable source with any standing to interpret and declare what citations mean or don’t.

As I have enumerated above, there is source after source after source that say that, accelerometers can not distinguish between gravity and inertial acceleration. If you persist with WP:refuse to get the point and tendentiously edit, Wikipedia and its policies will deal with that when the time comes. Greg L (talk) 16:33, 23 January 2009 (UTC)

• Yeah, I think you'll find that quote cuts both ways, much more your way than ours.- (User) Wolfkeeper (Talk) 16:58, 23 January 2009 (UTC)