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I never thought of this before but when an $8^{th}$ standard student asked me to explain what momentum is, I simply said that it the amount of motion contained in a body and tried to explain that how a truck moving at $10\ \mathrm{m\ s^{-1}}$ has more motion than a cat with the same speed (if you don't believe that such a definition even exists then look here ). But he asked that what's the difference between the speed or velocity and motion, if any? I think there must be some slight difference in how we define these. I particularly know of speed and velocity though not motion. (For me it seems that speed rather than being a different quantity is itself a quantifier of motion and hence am a bit jumbled up)

So if possible can someone clarify:

  • What motion is?

And in due process contrast it with other quantities such as speed, etc.

  • Speed and the quantity of motion (momentum) are just measurable properties of Motion, which is the physical phenomenon itself. – Swike Dec 25 '19 at 17:11
  • I'm puzzled, because I would never say that momentum is the amount of motion a body contains. Momentum is a well-defined physical quantity, while motion is similar to movement: It describes that the location of the body is not a constant in time. However, since english is not my primary language, I am happy to learn. – Semoi Dec 25 '19 at 17:36
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    The equivalence between momentum and "quantity of motion" is not common. To me, the confusion between "motion" and "quantity of motion" is ... confusing. Perhaps "quantity of motion" is better left unused, unless it can be clearly defined. Even then, given that it's not common and seemingnly adds nothing to understanding, I would personally not use it. – garyp Dec 25 '19 at 17:36
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    Motion is dependent on a reference frame The cat and truck have different amounts of kinetic energy see; https://en.wikipedia.org/wiki/Motion and https://en.wikipedia.org/wiki/Kinetic_energy – Adrian Howard Dec 25 '19 at 17:38
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    Physics is more about predicting experiment results and improve technology. "What is motion" is irrelevant to these - even you know exactly the answer, there won't be more experiments predicted or better technology. – jw_ Dec 26 '19 at 03:33
  • @jw I don't agree with that. Physics isn't just about predicting experiments and improving technology rather also about understanding the nature and getting the sense of how the universe works (and I assure you there are, were and would be many such physicists who strive to achieve such). For example we have quantum mechanics and general relativity in our hands & yet we haven't stopped yet and are trying to figure out what the laws there mean (especially QM). Also another example of what I said would be the LHC itself where people are trying to discover those tiny pats of the universe just... –  Dec 26 '19 at 07:33
  • @jw ... for the sake of their interest in understanding the nature. –  Dec 26 '19 at 07:34
  • In Portuguese, "momentum" is quantidade de movimento, which translates literally to "quantity of motion". – rafa11111 Dec 26 '19 at 13:30

7 Answers7

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I don't like these definitions of momentum. They may try to give an intuitive explanation but some will still find it hard to understand why these definitions correspond specifically to $m\vec{v}$ in Newtonian mechanics.

The truth is that momentum is just a vector that we discovered that it is conserved when there is no external force on the system. It is a helpful definition because it tells you exactly its purpose in physics: to be a quantity that we know that must be conserved, and by taking it into account, we can gain some insight about what happens in systems.

This is important, because in some situations, the momentum isn't $m\vec{v}$ but it is still conserved. For example, to calculate the momentum of a light beam with energy $E$, the correct formula is $|\vec{p}|=\frac{E}{c}$ ($c$ is the speed of light).

One can of course try to find an intuitive explanation for the Newtonian formula, but at the end of the day the answer will always miss a bit the general idea of momentum.

Ofek Gillon
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    Pardon me if what I say is stupid, but why E/c not mv (with v = c and m = E/c^2, obviously)? – Dan M. Dec 26 '19 at 16:51
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    E/c^2 * c = E/c, that's why! – simon at rcl Dec 26 '19 at 19:27
  • @DanM. Because a light beam has no mass - $m=0$. Which is why the $mv$ definition of momentum doesn't work in relativity. – IanF1 Dec 26 '19 at 21:59
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    @IanF1 a light beam has no rest mass, however, the full relativistic mass of a light beam (just as for any other system) includes not only the rest mass but is also increased by all energy in it. For all intents and purposes (inertia/momentum and gravity) the full mass that matters of a light beam is not zero but E/c^2. – Peteris Dec 26 '19 at 22:27
  • @simonatrcl that's what I said. There is is missing before not in my question and not a comma. I.e. I;m not sure why "momentum is mv" doesn't apply here (according to Ofek Gillon) when it seemingly does. – Dan M. Dec 27 '19 at 10:43
  • @DanM. what you're doing is an abuse of notation (don't get offended, it is an actual term: https://en.wikipedia.org/wiki/Abuse_of_notation ). Also, you can't say the mass of the light beam is $\frac{E}{c^2}$. Furthermore, I'm not sure that this abuse of notation will hold in a dielectric medium. – Ofek Gillon Dec 27 '19 at 10:50
  • @OfekGillon maybe. I'm not a physicist, although I wouldn't be surprised if it's still a correct transformation/way to look at it with a few steps omitted (it's Newtonian after all, while light would probably require some relativistic adjustments). – Dan M. Dec 27 '19 at 11:01
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I believe the answer is actually very simple. To be "in motion" simply means to have $\vec{v} \neq 0$. Nothing more, and nothing less. (Thus, as a consequence being in motion implies that speed $\neq 0$)

In physics we don't usually talk about objects having "more" or "less" motion. It's more of a yes/no thing. I'm not convinced that the truck has "more motion" than the cat.

If you're interested in trying to define momentum for students without resorting to the "it's a conserved quantity so it's useful" argument (I agree that this is often at too high of a level for some introductory physics students), you might want to say something like this: For a bunch of different objects moving at the same velocity, momentum describes how hard it is to alter the velocity of each object. Let's say 10 mph, to the north. Even if you try with all your might you may not be able to change the speed of a train traveling at 10mph. Thus it has large momentum. Meanwhile, you can change the speed of a feather traveling at 10mph with a simple flick of your finger. Thus its momentum is much lower.

Bunji
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    Sorry that I can't accept two answers simultaneously though your answer was quite helpful. +1 –  Dec 26 '19 at 06:54
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    "momentum describes how hard it is to alter the velocity of each object" No.. that was inertia.. – GPS Dec 26 '19 at 09:48
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    @GPS Since in the answer the velocity is fixed, they are the same. It might be better to say it is how hard it is to change the velocity, at fixed velocity (i.e. it is proportional to inertial mass) and for fixed mass it measures the speed (i.e. it is proportional to velocity). But that is basically the same as saying p = mv, and is only entirely true classically and for normal matter... – Graipher Dec 26 '19 at 10:13
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    @GPS I think by inertia here it means inertia of motion. –  Dec 26 '19 at 15:36
  • @GPS careful not to fall into the trap of saying that objects “have inertia.” The Law of Inertia is a law — not a property that can be possessed. – Bunji Dec 26 '19 at 16:33
  • @Bunji I agree with Graipher, you just described the Newtonian definition of inertial mass (and saying it needs to be proportional to the velocity too is just saying "we'll define the momentum to be mv". I think if there is an intuitive explanation you're looking for, it's ok (although what I wrote in my answer) but it will be quite challenging. When people ask me what energy is, I also had the same kind of trouble until I read Feynman's explanation in his lectures and it really explains it better (but that's my opinion). I recommend it very much, and would be happy to see a better one. – Ofek Gillon Dec 26 '19 at 23:17
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So if possible can someone clarify:

What motion is?

Momentum is a vector quantity. So "motion" in momentum is velocity $v$ and momentum is $mv$. Velocity is a vector quantity having magnitude and direction. The magnitude of the velocity is often referred to as speed. The speedometer in your car gives you the speed of the car. The speedometer does not tell you what direction the car is going. To know the velocity of the car, and thus its momentum, you need in addition the direction the car is going.

Hope this helps.

Bob D
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It's an interesting feature of English that it doesn't have non-technical terms for distinguishing between velocity, momentum and kinetic energy. In non-technical language "speed" subsumes all three concepts. German for instance has native words for all three: Geschwindigkeit (velocity), Schwung (momentum) and Wucht (kinetic energy). Since the concepts are native to the language, people learn the difference between these concepts as kids und understand it intuitively:

  • velocity is just how fast something moves between places
  • momentum is how much something moves you if you catch it (say, catching a tennis ball vs catching a medicine ball)
  • kinetic energy is how much it hurts when you don't catch it (say, getting hit in the chest by a tennis ball vs a medicine ball)

This intuitive understanding can of course also be explained to English speakers, they'll just have to be convinced that these are really different concepts. On the other hand, calling "momentum" the "amount of motion" doesn't help in developing the instinctive distinction between these three concepts. It's just using a different word without adding information.

tobi_s
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I think you want to explain it intuitively, so what I am going to write is how I understand it intuitively. First, I accept 3 things.

1)An intuitive notion of what POSITION is.

2)An intuitive notion of what MASS is.

3) I would intuitively expain what FORCE is, a proportion: the harder you push an object, the more force you apply.

Distance is the relative position between two objects. Speed determines how fast and in what way -is it getting bigger or smaller- that changes. Momentum is mass times velocity and more importantly, it is measures how much force has been applied to the object. To get great momentum you are required to apply great force.

In this context I would describe MOTION as the process by which we determine the position of the object. Quantity of motion is a physical quantity, usually diferent from the 3 'fundamental' concepts I described above, but directly described by them.

I hope this helps :)

Quantumight
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"Motion" is a binary quantity -- something is either in motion or not (stationary). (You might want to introduce simple relativity and "frame of reference" here with a simple car example: the driver's coffee cup in the cup holder is stationary in the driver's frame of reference but not in the usual "surface of earth" reference frame.)

"Speed" is motion quantified. Distance unit per time period.

"Velocity" is motion both quantified and with direction included. It might be interesting to note that "direction" here is ambiguous as to 1, 2, or 3 dimensions.

"Momentum" is motivated by the observation that differently massed objects behave differently in collisions with other objects. Heavier objects have "more effect" than lighter-weight objects travelling the same speed. And in fact (Newtonian) physics has determined that as to "constant effect", mass and speed are exactly inversely proportional -- doubling the mass or doubling the speed causes the same change in effect. So it is a useful quantity to multiply mass by speed -- voila Momentum.

Also to note, unlike "speed" vs. "velocity", "momentum" is ambiguous between the scalar and vector (direction included) variety.

Also to note, the "conservation of momentum in closed systems" that (Newtonian) physics has determined. As opposed to, say, velocity.

Jeff Y
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What motion is?

Well, it's a bit subjective matter. But to me it is more related with body trajectory in some reference frame

Agnius Vasiliauskas
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