Talk:Momentum

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lacks references, but otherwise looks like a good article! Snailwalker \| talk 00:17, 21 October 2006 (UTC)

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Help with this template Comments: edit – history – watch – refresh lacks references, but otherwise looks like a good article! Snailwalker \| talk 00:17, 21 October 2006 (UTC)

1 Why cite poor books
2 Earlier talk archived at
3 Notation
4 Impulse - step function?
5 removed one of the sections on impulse
6 Lede
7 Formatting of equations
8 Canonical momentum X mechanical momentum
9 Momentum operator in QM
10 Photon momentum
11 Possibly missing information from the page
12 Generalized and canonical momenta vs Newtonian momenta
13 Link to self-published paper?
14 What's a Hartman?
15 Use of p for momentum

[edit] Why cite poor books

I do not understand why at the end of the articles books like serways Physics for Scientists and Engineers are cited and books like Kittel or Kleppner Mechanics are not if the last ones are better.

 Kids I think don't know what momentom is, and this is adult language.  My perspective is getting fun with the writing!

--69.248.244.83 (talk) 23:12, 10 December 2007 (UTC)Mary Michelle

[edit] Earlier talk archived at

Talk:Momentum/archive1

[edit] Notation

The notation in the momentum page that used to describe collisions is different than that that on the page for elastic collision. The former uses an f subscript, the latter uses the prime (') superscript. I like the latter, it is more compact.

[edit] Impulse - step function?

On this page it says: impulse is a step function change in momentum.
By definition, an impulse happens instantaneously,(but it may transfer the same amount of momentum as a slower interaction does)
An (ideal)impulse is "an infinite force applied for zero time but with finite momentum change"
Impulse is plain and simple an instantaneous change in momentum (a step function change)

--Light current 02:45, 17 April 2006 (UTC)

Huh? Can you provide a reference for this? The impulse page says nothing about a step function. In my textbook (Sears, Zemansky, and Young), impulse is defined as force x time, and it says nothing about instantaneous or ideal impulses. Another book defines an impulse function as a synonym for step function, but that has nothing to do with momentum. Pfalstad 14:33, 17 April 2006 (UTC)

No. I didnt put it in! but Ive always thought of an impulse as a step change in momentum. Hence my statements above. Thats what this article page says. Are you saying that's wrong? If so, please feel free to correct the page and my misunderstanding of the definition of impulse will be corrected by your doing so ;-)--Light current 15:44, 17 April 2006 (UTC)

I'm saying that's wrong. Pfalstad 19:25, 17 April 2006 (UTC)

Looks like the word has different meanings in signal theory/math and mechanics! How interesting! I cant believe that mech engs dont use the dirac delta function in working out collisions taking v short times. But we'll never know as we dont seem to meet many mech engs here. I suppose we'll have to rely on physicists. Oh well...--Light current 15:51, 17 April 2006 (UTC)

When working out collisions, esp. when treating them as instantaneous events, I don't think you deal with force at all. At least not in freshman physics. You deal with conservation of momentum and kinetic energy. No need to introduce impulse or delta functions. Pfalstad 19:25, 17 April 2006 (UTC)

You're the boss on this one!--Light current 19:47, 17 April 2006 (UTC)

What about a hammer striking a nail (when its done properly!)? The time of contact is very small (cos the hammer bounces off) yet most of the hammer's momentum is transferred to the nail. Do we not call this an impulse? -- A delta function?. Have mechanical engineers not heard of this yet? Shame!--Light current 20:48, 17 April 2006 (UTC)

Well, looking up stuff about impulse very quickly, I can't find anything about instantaneous change. Defining it that way makes no sense to me. In the hammer example, yes thats an impulse, but not because of the "short" amount of time, its simply because of the change in momentum - the momentum tranfer. I'll change the part about impulse. User:fresheneesz 68.6.112.70 09:46, 18 April 2006 (UTC)

I fixed it, I reccommend we remove the disputed facts tag - but I'll leave that up to you guys. User:fresheneesz 68.6.112.70 09:52, 18 April 2006 (UTC)

Ok Its not disputed now so Ill remove tag--Light current 13:11, 18 April 2006 (UTC)

Funny thing, today, my ECE prof mentioned an "impulse" that is infact a step functionish sort of thing. However, it is distinctly different from the "change in momentum" impulse. Look at delta function - they give a maybe incorrect reference to a unit impulse function. I think impulse can mean instantaneous change, but not in the context of momentum. Fresheneesz 05:04, 19 April 2006 (UTC)

Ahh, actually look up impulse response, and that gives more insight (but not much more). The disambiguation page for impulse gives it - I wonder if the page impulse should be turned into a full disambig page. Fresheneesz 05:09, 19 April 2006 (UTC)

"Impulse" in the classical physics sense is different than "an impulse" in the electronics sense. In the physics sense, you don't say "an impulse is...". You say "the impulse of this force is equal to..." Or you measure impulse. You don't measure an impulse. I think there are a bunch of problems with this section; it's confusing the electronics meaning with the physics meaning. Unfortunately impulse is used so little in physics that my sources don't talk much about it. (I'll let LC indent this comment appropriately.) Pfalstad 05:30, 19 April 2006 (UTC)

I think all mention of impulse should be removed, and instead just have a link to impulse in the "see also" section. It's easier than cleaning it up. Impulse is just not important enough to warrant such a long discussion in this article. My physics text goes on and on about momentum but barely mentions impulse, and I can't remember using it since freshman physics. Pfalstad 05:34, 19 April 2006 (UTC)

I actually never used impulse, not once. Its really a useless concept as far as I'm concerned. I'm going to merge the info on this page with the page on impulse - but I'll leave it up to someone else to actually delete it and put it in the see also section. Fresheneesz 06:58, 19 April 2006 (UTC)

[edit] removed one of the sections on impulse

I removed the larger section on impulse beause it almost fully duplicates the article impulse. Fresheneesz 08:22, 19 April 2006 (UTC)

[edit] Lede

Restore balance and focus of para to the more conventionally accepted one. --Light current 15:12, 20 April 2006 (UTC)

[edit] Formatting of equations

Whats with the fancy formatting? Is this in th MoS?--Light current 15:14, 20 April 2006 (UTC)

I doubt it. I originally did it to utilize more page space, and separate the variable definitions from the rest of the text. Now its just for the ladder reason, I think it works well. You don't like it? Fresheneesz 02:53, 21 April 2006 (UTC)

[edit] Canonical momentum X mechanical momentum

I want to put forth only a thing: is there really a distinction about canonical momentum as opposed (though related to) mechanical momentum, as we could see in Sakurai,J.J - Modern Quantum Mechanics page 130 (between formulae 2.6.22 and 2.6.23) 1994?

[edit] Momentum operator in QM

The article states:

For a single particle with no electric charge and no spin, the momentum operator can be written in the position basis as

$\mathbf{p}={\hbar\over i}\nabla=-i\hbar\nabla$

where $\nabla$ is the gradient operator. This is a commonly encountered form of the momentum operator, though not the most general one.

Why the qualification about not having spin or electric charge? Surely the formula is valid more generally? If not then let's say what the more general formula is (perhaps we wish to mention conjugate momenta?). --Michael C. Price ^talk 13:59, 6 October 2006 (UTC)

[edit] Photon momentum

In the article it states that $\vec p = m\vec v + q\vec A$ , accuratly represents a particle's momentum, but this still doesn't cover photons because photons have no net charge! Am I missing something here?

Is it really true that $\vec p = m\vec v + q\vec A$ accuratly represents a particle's momentum? I thought this was a replacement done in Schroedinger's equation to incorporate the (electric) potential energy and Lorentz force (from motion in a magnetic field).

Since photons have no charge, the $q \vec A$ term is zero for a photon. This is the same as saying that photons don't interact with the electromagnetic field. HEL 03:13, 5 November 2006 (UTC)

Not to mention the fact that the $m\vec v$ term is zero as well since the photon has no mass. No, the $\vec p = m\vec v + q\vec A$ holds only for a charged massive particle. PAR 03:35, 5 November 2006 (UTC)

The photon's momentum follows from the properties of the electromagnetic field, for which it is the quantum. See e.g. Jackson for a discussion of field momentum. Bkalafut 05:19, 28 June 2007 (UTC)

[edit] Possibly missing information from the page

The article at present describes the effects of momentum. But at present it doesn't explain what it actually is. (For example knowing the effects of smoke inhalation is different from knowing the chemical composition of CO2 and the biology behind breathing.) Would it be more accurate (and hence saying in the article) that this 'law' is actually a 'theory that past observations of CoM apply to all things'? This is an idea I found in a popular science book. (Actually, no one really knows what momentum is, we just understand its effects.) Is it an accurate thing to say, or was something lost in the translation? ANTIcarrot 14:16, 13 December 2006 (UTC)

I think that the description on this page is misleading.

Note: Newton's First Law of Motion is often called the Law of Inertia. It reads, "An object will not accelerate unless acted on by an unbalanced force". Another way to phrase it would be, "An object will stay at constant velocity unless acted upon by an unbalanced force".

The description of momentum on this page sounds a lot like Newton's First Law of Motion. "the tendency of an object to continue to move in its direction of travel, unless acted on by a net external force" To me, this sounds more like a description of an object's inertia: "how difficult it is to accelerate the object", or "the object's natural tendency to resist changes in its motion".

I teach that an object's momentum is a measure of how difficult it is to stop the object. This depends on two factors multiplied togther the object's inertia and the object's speed.

If an object has more inertia (natural tendency to resist changes in its motion), then it will be more difficult to stop.

If an object is moving faster, then it will be more difficult to stop.

I hope this is clear and can be used to improve the page.

--Unksnogan 21:35, 3 January 2007 (UTC)

Correct me if i am wrong, but inertia cannot be multiplied as it is not a quantifiable value. Perhaps you mean the objects mass? 59.167.18.132 14:27, 28 August 2007 (UTC)

[edit] Generalized and canonical momenta vs Newtonian momenta

This article doesn't seem at all to cover generalized momenta--the partial derivative of the Lagrangian with respect to velocity--and thus completely misses the momentum of a charged particle interacting with an electromagnetic field, or angular momentum. Is there a reason for this? Bkalafut 05:16, 28 June 2007 (UTC)

[edit] Link to self-published paper?

I removed a link to a self-published paper on "Unified Momentum Theory", since it has all the earmarks of a crank/crackpot theory. (Its own webpage with the word "God" in the title, author's yahoo email address in the paper, etc.) —Preceding unsigned comment added by Langphysics (talk • contribs) 09:57, 16 October 2007 (UTC)

[edit] What's a Hartman?

I've removed the paragraph that claimed the unit of momentum was a Hartman. It was posted by some guy at my high school, naming it after one of our physics teachers.

Bold face Killa 20:45, 26 October 2007 (UTC)

Hey but there does need to units for momentum like everything else. Come on cant the scientists be lazy by having a simplified version of kg*m/s. Who wants to wright that out each time they solve for momentum. There are lots of derived units that just make it easier in our lives. So a Hartman is a time saver. --Cybertiberius (talk) 23:35, 26 October 2007 (UTC)

It just makes sence to have a unit for momentum anyways. --Cybertiberius (talk) 23:42, 26 October 2007 (UTC)

A unit is what you make it. I'm gonna use it. You know how the units of inch, foot, yard, etc. originated? The length of the king's body parts. Just because some group of scientists says it isn't an official unit doesn't mean it can't be used...it just shouldn't be used anywhere where official SI units are required. People make up units all the time. Hell, in math class, if you've ever used something like 'Let y = (2x^3)', you're inventing a unit...though only temporarily. 209.160.28.45 00:19, 27 October 2007 (UTC)

The Hartmann is the unit of wikipedia banning.WolfKeeper 00:48, 27 October 2007 (UTC)

So it's cool for kings to randomly decide what a measurement is, but experiments to create one democratically must be quashed? I mean, hell, even the SI system was created by a king. Of course, I do realize you have a job to keep things truthful. What if it is phrased as 'There is a movement to create the Hartman as the unit of momentum'?

71.61.37.175 01:13, 27 October 2007 (UTC)

That's not a notable point of view quoted by a reliable source.WolfKeeper 01:21, 27 October 2007 (UTC)

How can there be sources if it is used everyday by physics teachers and not on websites yet? It is used commonly but it is one of those things that is hard to source, like how does one cite something that is used by all? That what the Hartman is like. --Cybertiberius (talk) 01:08, 28 October 2007 (UTC)

"Sucks to be you then".WolfKeeper 05:13, 28 October 2007 (UTC)

all of its all lies —Preceding unsigned comment added by 74.249.94.240 (talk) 22:41, 6 December 2007 (UTC)

[edit] Use of p for momentum

Removed from article: "The origin of the use of p for momentum is unclear. It has been suggested that, since m had already been used for mass, meter, and -milli, the p may be derived from the Latin petere ("to go") or from "progress" (a term used by Leibniz).^{[citation needed]}"

I would love to see more cultural lore in these articles, and I was the one who expanded the history section and answered the template on Newton. I have tried my level best to find the origin of the use of p and I cannot do it! For me not to be able to find it is saying something. It appears in the late 19th century without a trace before then. Maybe it means P for the point at which the force applies or p for the power of momentum. The earlier writers generally did not assign a variable to momentum, they used descriptive words, such as momentum, and none of those seem to fit p. Well, let's keep looking, but in case we are wrong, let's not mislead the public, so I moved the statement to here. There must be a first use of p that caught on or else it would not be a standard variable. Whether it was considered important enough to note is another matter, so we would at worst case scenario be searching every physics book in the world since 1689 to find out the earliest who used p! Whew.Dave (talk) 14:39, 23 February 2008 (UTC)