Talk:Kinetic energy

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Kinetic energy as a relative concept.

(Follow up from a previous topic)

One aspect of the current article concerns me: possession of kinetic energy. As I heard it put one time: energy is not a magical fluid added and removed from objects. Rather it is a relative property, useful only by comparison. Objects have different amounts of kinetic energy depending on the reference frame. We humans assign kinetic energy to objects based on our analysis of the object's velocity relative to our chosen reference frame.

In one sense this is obvious: velocity is relative. Object's don't possess velocity, thus they cannot possess kinetic energy. The difficult part is presenting this naturally and clearly for an introductory audience and with sufficient reliable references.

I think such changes are important for this kind of article as noted in WP:OVERSIMPLIFY.

My proposal is to first develop a short section explaining 1) why kinetic energy is not an object property and 2) why assigning kinetic energy to objects is so darn useful.

And second to make small changes to the article to reduce the dependence on the possession model. For example, rather than

the kinetic energy of an object is the form of energy that it possesses due to its motion

we might say

the kinetic energy of an object is the form of energy associated with its motion.

I will look for appropriate references first. Johnjbarton (talk) 01:04, 9 September 2023 (UTC)[reply]

Unfortunately I don't have the right books needed to add references to this article. Johnjbarton (talk) 22:39, 9 September 2023 (UTC)[reply]

Walker, Halliday, and Resnick (2014) use the word "associated":

Kinetic energy K is energy associated with the state of motion of an object. The faster the object moves, the greater is its kinetic energy. When the object is stationary, its kinetic energy is zero. (page 151)

However, on Wikipedia, alternatives should be noted and cited.

Thus, I would reword the lede to use the word "associated" and note that Jain (2009) uses the word "possessed".

--50.39.109.119 (talk) 20:59, 18 November 2024 (UTC)[reply]

Feynman uses "possess" in this sentence:

The fact that the electromagnetic field can possess momentum and energy makes that field very real, ... (Feynman Lectures, Volume I, Section 10–5)

Found with a web search for "possess site:feynmanlectures.caltech.edu".

--50.39.109.119 (talk) 03:02, 19 November 2024 (UTC)[reply]

Do you have evidence that these authors are describing different physical models? I say "no", they are just using words appropriate to the goals of their readers. Jain is writing for engineers who need to apply KE to practical problems. Assigning object KE is a routine part of newtonian mechanics. The total KE of a system is partitioned among the objects as part of leveraging conservation of energy. That does not mean KE is an intrinsic property of the objects, but rather a form of bookkeeping.

Resnick is writing for physics students. Thus "associated" is used to avoid setting an incorrect concept in the minds of students. Note that the article says: "Thus, the kinetic energy of an object is not invariant." The summary in the lede should not contradict the article.

This ref has some information on the complexity of KE historically. (It does not address this issue directly)

Antippa, Adel F. "On the concept of kinetic energy." Canadian Journal of Physics 78.10 (2000): 883-899.[1]

Johnjbarton (talk) 18:01, 19 November 2024 (UTC)[reply]

WHR, Feynman, and Fermi are all physicists, and they are all reliable and verifiable sources.

All that needs to be acknowledged in the article is that some authors use "associate" and some authors use "possess".

As for the ambiguity of the concept of energy generally, see:

Energy, the Subtle Concept : The Discovery of Feynman's Blocks from Leibniz to Einstein by Jennifer Coopersmith (Oxford University Press, 2015).

(The phrase, "Feynman's Blocks", refers to the Feynman Lectures, Volume I, Section 4–1.)

--50.39.109.119 (talk) 19:00, 19 November 2024 (UTC)[reply]

Yes, well the punchline is in Feynman:

"It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount."

KE can't be an intrinsic property, we don't even know what it is. Johnjbarton (talk) 22:57, 19 November 2024 (UTC)[reply]

Note that WHR and Jain are explicitly referring to classical kinetic energy, in which case no frame of reference is considered. Specifically, WHR never refers to a frame of reference in the section on "What is energy?" (pp. 149-150) --50.39.109.119 (talk) 21:48, 19 November 2024 (UTC)[reply]

Fermi:

Ionization phenomena by collision can be explained in a very similar way. It is known that all atoms beyond the limit of their principal series exhibit, both in absorption and emission, a continuous spectrum, corresponding to the transition of the valence electron to the state in which it is ionized and furthermore possesses some kinetic energy.

On the Theory of Collisions between Atoms and Electrically Charged Particles, note by Enrico Fermi, 1924 (English translation 2001)

Both translators are scientists.

Found with a Google Scholar search for "possess author:enrico author:fermi".

--50.39.109.119 (talk) 06:40, 19 November 2024 (UTC)[reply]

The phrase "possesses kinetic energy" is used extensively in scientific papers. Presumably, the authors are assuming a particular frame of reference. --50.39.109.119 (talk) 22:40, 19 November 2024 (UTC)[reply]

Sure but they are using it like this:

Any mass, molecule, atom, or electrical unit, in a state of motion, possesses Kinetic Energy.

This is simply the definition. Once you have identified an mass and its "state of motion" then presto it has KE by definition.

But this is curious "possession". I possess a car and $20. But how much KE do I possess? Zero because I am sitting still? A great deal because I am hurtling through the Milky Way along with the Sun? The state of motion and thus the KE depends upon what problem I seek to solve. If you are riding on a train vs about to be hit by a train you are solving very different problems and the KE of the train is correspondingly different for you. Johnjbarton (talk) 23:05, 19 November 2024 (UTC)[reply]

Semi-protected edit request on 16 November 2024 (Einstein, The Meaning of Relativity)

This edit request has been answered. Set the |answered= or |ans= parameter to no to reactivate your request.

In the section on Relativistic kinetic energy, I would like to add this note citing Einstein himself for the Taylor expansion of the relativistic energy:

Although Einstein does not explicitly refer to a Taylor expansion, in equation 45 of The Meaning of Relativity, Einstein "develop[s] ${\displaystyle E}$ in powers of ${\displaystyle q^{2}}$ ". In equation 45, ${\displaystyle q}$ is the magnitude of the velocity ( ${\displaystyle q}$ is defined on page 38), and the velocity of light is the unit (page 46).

The note would be placed after the first mention of "Taylor expansion" in the article:

"This is done by binomial approximation or by taking the first two terms of the Taylor expansion<<insert note here>> for the reciprocal square root: ..."

-- 50.47.156.69 (talk) 22:24, 16 November 2024 (UTC)[reply]

Thank you for this source. I encourage you to register for an account and contribute directly. Johnjbarton (talk) 23:16, 16 November 2024 (UTC)[reply]

Thanks for adding the Einstein reference. I have converted it to a "Cite book" template, which adds a link to the exact page being referenced:

{{cite book |last1=Einstein |first1=Albert |title=The Meaning of Relativity: Four Lectures Delivered at Princeton University, May, 1921 |date=1922 |publisher=Methuen & Company Limited |pages=51-52 |url=https://en.wikisource.org/wiki/Page%3AThe_Meaning_of_Relativity_-_Albert_Einstein_(1922).djvu/63}}

That renders like this:

Einstein, Albert (1922). The Meaning of Relativity: Four Lectures Delivered at Princeton University, May, 1921. Methuen & Company Limited. pp. 51–52.

NB: My IP address changed.

--50.39.109.119 (talk) 01:20, 17 November 2024 (UTC)[reply]

Maybe we should link the free Google books version which has all the pages: https://www.google.com/books/edition/The_Meaning_of_Relativity/0nIxAQAAMAAJ?hl=en&gbpv=1&dq=approaches&pg=PA51&printsec=frontcover Johnjbarton (talk) 01:29, 17 November 2024 (UTC)[reply]

Thanks for updating the article with the cite book template. On the Wikisource page, there are navigation arrows at the top left corner. However, I agree that the Google books version would be a better one to link, because it is easier to navigate. Also, search terms are highlighted in yellow. Unfortunately, the page break means that readers will have to scroll down one page to see the highlighted text. --50.39.109.119 (talk) 02:09, 17 November 2024 (UTC)[reply]

Thanks for updating the link. It works fine. Interpreting what Einstein means by "q" took me some effort, which is why I added a second sentence to my original suggestion. Since E. is a primary source, it might be a good idea to add a secondary source making the same point about the classical kinetic energy being the limit at low speeds. --50.39.109.119 (talk) 02:40, 17 November 2024 (UTC)[reply]

Books that are cited over 5000 times are great sources. Nevertheless a newer source would be great. I'll note that a much bigger problem in this article is large chunks of text with no sources. Johnjbarton (talk) 02:55, 17 November 2024 (UTC)[reply]

Here is a newer source (Robert Resnick, Introduction To Special Relativity (Wiley, 1968)) for the relativistic kinetic energy as a series. WolframAlpha computes the Taylor series used in the expansion. --50.39.109.119 (talk) 04:14, 17 November 2024 (UTC)[reply]

Thanks for adding the Resnick ref. It might be useful to make it a named ref and then cite page 123, equation (3-18b), for the energy-momentum relation at the beginning of the Relativistic kinetic energy section.

Another addition would be an "author-link" parameter in the cite book template:

{{Cite book |last=Resnick |first=Robert |author-link=Robert Resnick |url=https://archive.org/details/robertresnickintroductiontospecialrelativitywiley1968/page/n131/mode/2up?q=%22kinetic+energy%22 |title=Introduction to special relativity |date=1968 |publisher=Wiley |pages=121-123 |isbn=978-0-471-71725-6 |location=New York |access-date=2024-11-17}}

Resnick, Robert (1968). Introduction to special relativity. New York: Wiley. pp. 121–123. ISBN 978-0-471-71725-6. Retrieved 2024-11-17.

--50.39.109.119 (talk) 08:55, 18 November 2024 (UTC)[reply]

Semi-protected edit request on 18 November 2024 (Halliday & Resnick, Physics)

It is requested that an edit be made to the semi-protected article at Kinetic energy. (edit · history · last · links · protection log)

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The article already cites Halliday & Resnick, Physics (1960), twice in the lead. I would like to add it to the section on Kinetic energy for non-relativistic velocity to support the first few sub-sections. The ref is already named, so it is easy to add.

"Cite book" template with updated citation to follow.

-- 50.39.109.119 (talk) 19:03, 18 November 2024 (UTC)[reply]

Cite book template with updated citation:

{{cite book |last1=Walker |first1=Jearl |last2=Halliday |first2=David |last3=Resnick |first3=Robert |author-link1=Jearl Walker |author-link2=David Halliday (physicist) |author-link3=Robert Resnick |title=Fundamentals of Physics |date=2014 |publisher=John Wiley & Sons, Inc. |location=Hoboken, NJ |isbn=978-1-118-23061-9 |pages=150-155 |edition=10th extended |url=https://archive.org/details/fundamentals-of-physics/page/150/mode/2up |access-date=18 November 2024}}

Rendered view:

Walker, Jearl; Halliday, David; Resnick, Robert (2014). Fundamentals of Physics (10th extended ed.). Hoboken, NJ: John Wiley & Sons, Inc. pp. 150–155. ISBN 978-1-118-23061-9. Retrieved 18 November 2024.

NB: Walker is now the lead author (per WorldCat).

--50.39.109.119 (talk) 19:40, 18 November 2024 (UTC)[reply]

Einstein's definition of kinetic energy

Einstein explicitly includes the rest energy in his definition of kinetic energy.

So the article is inconsisent with Einstein. That inconsistency should be explicitly pointed out in a note, or the article should be made consistent with a reliable source, such as Einstein himself.

See: Relativity: The Special and General Theory (1921), page 53.

NB: Different editions have different pagination and slightly different titles.

--50.39.109.119 (talk) 22:28, 20 November 2024 (UTC)[reply]

Resnick (1968) excludes the rest energy from his definition of kinetic energy:

The total energy of the particle (Eq. 3-17) is the sum of its rest energy* and its kinetic energy. (page 122)

So the article needs to acknowledge that different reliable sources define relativistic kinetic energy differently.

--50.39.109.119 (talk) 22:56, 20 November 2024 (UTC)[reply]

In his 1905 paper, On the Electrodynamics of Moving Bodies, Einstein introduces "the energy of motion W", which excludes the rest energy.

--50.39.109.119 (talk) 23:47, 20 November 2024 (UTC)[reply]

Relativistic kinetic energy is discussed in the article in the section appropriately named Kinetic Energy#Relativistic kinetic energy. Einstein is indeed cited in that section. Johnjbarton (talk) 00:25, 21 November 2024 (UTC)[reply]

Einstein is cited in the "Low speed limit" section with a ref that is *different* from Relativity: The Special and General Theory. Please look at the sources I linked and at this *unsupported* equation from the article:

E_{K}=E-m_{0}c^{2}={\sqrt {(p{\textrm {c}})^{2}+\left(m_{0}{\textrm {c}}^{2}\right)^{2}}}-m_{0}c^{2}

The equation subtracts the rest energy,

m_{0}c^{2}

.

That subtraction needs to be justified by citing a source, such as Resnick (1968), which you previously added. Indeed, the same page can be cited.

As for Einstein, you must not have actually looked at the page I linked, because you completely ignored my point that E. includes the rest energy in his formulas for the kinetic energy.

--50.39.109.119 (talk) 01:39, 21 November 2024 (UTC)[reply]

Oh I looked at the page in the book on relativity. What I saw was a discussion of "relativistic kinetic energy". It's a different thing, as you have noted, because it includes rest energy. Johnjbarton (talk) 02:06, 21 November 2024 (UTC)[reply]

Thanks for looking at that. My suggestion for the article would be to cite Resnick (1968), who does not include the rest energy in the kinetic energy, which is consistent with the article. And to add a footnote saying that in Relativity: The Special and General Theory, Einstein does include the rest energy in the kinetic energy (page 53).

As for Einstein's 1905 paper, in which he uses the phrase "energy of motion", that phrase is a synonym for "kinetic energy" (Rigden, 2009, p. 113).

--50.39.109.119 (talk) 03:16, 21 November 2024 (UTC)[reply]

no source cited for the phrase "energy of motion"

The article uses the phrase "energy of motion" without citing a source, such as Rigden, 2009, p. 113. -- 50.39.109.119 (talk) 03:25, 21 November 2024 (UTC)[reply]