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November 24

Work: Where does it go?

Hello everyone, I'm taking AP physics and have questions. Imagine you lift a 2 kg ball up one meter. You do positive work on the ball because the fore you apply is in the direction of motion, where as gravity does negative work because it's force is in the opposite direction. It's pretty easy to prove using the work energy theorom that the work done by both these forces are equal. Since the total change in kinetic energy from when the ball is on the ground to when you're done lifting it up is zero, the forces must be equal and opposing. Therefore, the total work done on the ball is zero. My questions is as follows: Where does the potential energy that the ball now has come from? If the total work done on it is zero, then how does he ball gain energy? If you say that the work done by you hand gives the ball it's energy, then where did the work of gravity go towards? By the law of conservation of energy, that work must be transformed into some sort of energy. Right? What am I missing here? 24.218.164.221 (talk) 01:39, 24 November 2023 (UTC)[reply]

Doesn't it take work to lift the ball? ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:38, 24 November 2023 (UTC)[reply]

It does. Gravity does the same amount of work in the opposite direction so the total work done on the ball by all forces is zero. 24.218.164.221 (talk) 04:08, 24 November 2023 (UTC)[reply]

Assigning work as being due to agents applying forces may sometimes be helpful for one's mental image, but it may also lead to confusion. One needs to choose a consistent approach. A common one is that in which the amount of work is equal to the decrease in potential energy, regardless of which agents the applied forces are ascribed to; see for example Work (physics) § Work and potential energy. From the point of view of the lifter, they have to inject the same amount of physical energy into the system (a ball subject to the force of gravity), whether they slowly lift the ball or quickly throw it into the air with just enough initial upward velocity that it goes up by one metre. The potential energy increases, so the work is negative. Since it is counterintuitive that the lifter's labour thus achieves negative work, sometimes the opposite approach (increase in potential energy) is preferred. --Lambiam 08:36, 24 November 2023 (UTC)[reply]

If you really want to know, the energy is stored in the gravitational field surrounding the ball and the Earth, not in the ball itself. But when dealing with balls moving in constant gravity, that's not a very useful approach. For the moment, choose between (A) gravity does no work, the energy is stored in gravitational potential energy in the ball and can be released from there; or (B) there is no gravitational potential energy, there is a gravitational potential, the derivative of which, when multiplied by a mass, gives a gravitational force, which can do work. Just pick the option that's most convenient.

BTW, if you move two opposing electric charges apart, the energy/work is stored in the electric field between the charges. If you move two aligned magnets apart, it's stored in the magnetic field between the magnets. In those cases it's common to deal with the energy of the field itself, but with gravity, this is rarely done because it's mathematically far more complex. PiusImpavidus (talk) 10:07, 24 November 2023 (UTC)[reply]

Would it be helpful for an explanation to use the equivalence principle and move OP and ball from the earth to a platform undergoing constant acceleration and observe from an inertial reference frame? The force and total mass of platform, ball, and OP we are accelerating remain constant, but as the OP lifts the ball it is displaced more and the platform less correct? fiveby(zero) 17:11, 24 November 2023 (UTC)[reply]

Conation

Morning Folks!! Is this article notable. The previous version was proded as it seemed to indicate that it was still being studied when its not. It was described as pseudoscience. The name Kolbe was mentioned in the previous articles. It also mentioned in this article. Hence the question. It is notable? Unusual request I know. I would like an opinion if possible. Thanks. scope_creep^Talk 11:10, 24 November 2023 (UTC)[reply]

@Scope creep A simple Google scholar search just for the word "conation" gives ~13,000 hits, so while the article may not be well written, the topic itself certainly seems notable. doi:10.5465/amr.2010.0128 has been cited 564 times, for example. Mike Turnbull (talk) 12:38, 24 November 2023 (UTC)[reply]

Excellent. That is ideal. scope_creep^Talk 12:47, 24 November 2023 (UTC)[reply]

Glans

I have phimosis, and I didn't realise I had phimosis until my thirties. Over the years I've mentioned this to a few men, and they've reacted with horror asking how I ever cleaned myself.

What I'm curious about is how men, in general, learn to wash. Our article on Foreskin states:

The foreskin is fused to the glans at birth and is generally not retractable in infancy and early childhood. [...] Retraction of the foreskin is not recommended until it loosens from the glans before or during puberty.

I think we were told in school, in passing, that we should wash our genitals, but nothing like the details of the whole business of retracting the foreskin right back and cleaning around the corona. Presumably their parents don't show them if it's not even possible until puberty. It's surely not an innate instinct. So if it's as basic a skill as they make it out to be, where are they getting it from?

Slightly embarrassing question, hence the throwaway account. Question610 (talk) 14:36, 24 November 2023 (UTC)[reply]

I can't speak for everyone, but I was instructed by my doctor while still very young, presumably after some problem with it. That's perhaps why my little brother was then circumcised. Shantavira|^{feed me} 20:34, 24 November 2023 (UTC)[reply]

Phimosis can vary in its degree of severity. For some men with the condition (like me) it is possible to retract the foreskin when the penis is flaccid (and wash), but not when erect.

Even when the condition is more marked, it may be possible to pinch the end of the foreskin shut temporarily while urinating, which briefly fills the internal space with urine and helps to flush out smegma. (Fresh urine is fairly sterile and, I have read, mildly antiseptic unless a bladder infection is present). Sources and degree of advice about these matters probably differs markedly according to individuals' circumstances.

Medical advice has also changed over the decades. In the 1950s, (at least some) doctors in the UK advised mothers to retract their baby's foreskin in order to wash the glans. I suspect this might actually be a contributary cause of phimosis in later life. {The poster formerly known as 87.81.230.195} 2.218.227.124 (talk) 05:11, 25 November 2023 (UTC)[reply]

Explicit medical advice is not allowed on the reference desks.

The following discussion has been closed. Please do not modify it.

You should probably get circumcised because everything you’re describing sounds abnormal and extremely unhygienic. Kojavak (talk) 23:18, 25 November 2023 (UTC)[reply]

@Kojavak: Wikipedia does not give medical advice, so you should not. Even if you happen to be a medical professional, that can't be verified and even if it could, it can't be in Wikipedia's voice.--Jasper Deng (talk) 23:34, 25 November 2023 (UTC)[reply]

I’m not a medical professional. I’m just saying the OP is abnormal and unhygienic. Kojavak (talk) 00:36, 26 November 2023 (UTC)[reply]

@Kojavak: Then it is even more important that you refrain from giving explicit medical advice on Wikipedia. This is a stated rule of the reference desks, per WP:RD/G: "The reference desk is not a place to seek professional advice on medical or legal matters, and responses that could be construed as such must not be given."--Jasper Deng (talk) 00:54, 26 November 2023 (UTC)[reply]

@Kojavak: I'd really rather not be called "abnormal and unhygienic" by some random stranger, just for asking a question. Yes, I have received medical advice, and I can wash now, though it's still uncomfortable. But I never properly washed until my thirties because I didn't know, and the question I asked was: how do people know? I'm not asking for advice about what I should do now. Question610 (talk) 01:30, 26 November 2023 (UTC)[reply]