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What EXACTLY is momentum?

  1. Jul 17, 2009 #1
    I don't want to know what it's like, I want to know what it IS.

    On an unrelated note, I read this excerpt from a Hyperphysics article on voltage;

    "Like mechanical potential energy, the zero of potential (of voltage) can be chosen at any point, so the difference in voltage is the quantity which is physically meaningful."

    What does that mean? It's really bothering me. :confused:
  2. jcsd
  3. Jul 17, 2009 #2


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    It's exactly mass times velocity.
  4. Jul 17, 2009 #3
    It is what it is. It's mass*velocity, it's the abstract quantity that is changed by force, which is conserved in a closed system.
  5. Jul 17, 2009 #4

    Vanadium 50

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    I think you'll have to expect to be disappointed. Suppose someone answers you. You can then point at the answer and say, "Yes, but what IS it?" Eventually you can reach a point where all that can be said is how "it" behaves.
  6. Jul 17, 2009 #5
    well I dont know what it is, but I know what it feels like to change it.

    If I say the potential energy of a bowling ball held 10 ft above the floor is 10 lb * 10 ft = 100 ft lb, we can use that to determine how fast the ball is going when it hits the floor (after I drop it). Now, if I tell you the floor is actually the 20th floor of the building, we can see that the "zero" is arbitrary. Voltage is the same, in the sense that the "zero" is arbitrary. Does that help any?
  7. Jul 17, 2009 #6
    Momentum is defined as p=mv where m is mass v is velocity and p is momentum.

    Newton's second law, F=ma, can be written as dp/dt this is because ma=m*dv/dt=d(mv)/dt=dp/dt

    That has a qualitative implication; the more momentum an object has, the harder it is to change it's motion.

    In Quantum Physics, de Brogle hypothesized that a particle was also a wave with the momentum defined as p=h/w where w is the wavelength and h is plank's constant which equals about 6.63*10^-34 J s.
  8. Jul 17, 2009 #7
    This happens most often to discussions that involve non-provable subjects. Not all physics concepts are so, at least I believe.

    Einstein once said, "Imagination is more important than knowledge."
    I take this to say, "To have a feeling of what momentum is is more important than to state p = mv."

    A more sensible way to describe what momentum of a mass body, which is quite common to hear, is "a mass's persistence to continue its motion." Which is similar to the concept of inertia.

    Imagine in empty space you have 2 objects. A drinking straw and a massive solid sphere of a bowling ball, both are drifting in space at the speed of a thrown baseball relative to you. Now if you try to stop either object by pushing them with the same force, you will have a much more difficult time in slowing the one with more mass (momentum directly depends on mass). Ask your self, compared to the straw, how does it feel the one that's heavier.

    What ever abstract feeling you get from that should be the answer to "what momentum IS"
  9. Jul 19, 2009 #8
    Well, considering we really don't know what either velocity nor mass is, nor energy in general, nor much of anything for that matter, it's not so surprising that we don't know exactly what momemtum IS, either. we do have good ideas about it behaves, however.

    Physics tends to describe what happens rather than why something happens or what something IS. As Richard Feynman is quoted as saying

    despite that fact he was quite expert at explaining g things in simple terms.

    We are very lucky we have mathematics...even luckier, I think, that our universe behaves as some math indicates....
  10. Jul 19, 2009 #9


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    As force is rate of change of momentum (Newton's 2nd law), there's a simple description of momentum. It's the constant force you'd need to apply to bring an object to rest in exactly one second (assuming momentum in SI units). Or, equivalently, it's the number of seconds it would take to bring the object to rest by applying a constant force of 1 Newton.
  11. Jul 19, 2009 #10
    As far as what it "is". I've heard momentum described as "the total amount of motion"
  12. Jul 19, 2009 #11
    What exactly is "force" and what is "time"? :confused:
  13. Jul 19, 2009 #12


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    I refer you to post #4.:smile:
  14. Jul 19, 2009 #13
    Momentum is the generator of translations:

    p = - i hbar d/dx ------->

    d/dx = i p/hbar

    psi(y) = [exp(y d/dx) psi]_{x = 0} = exp(i y p/hbar)psi(x=0)
  15. Jul 20, 2009 #14
    It is my understanding that momentum, to this day, is a mystery.

    That is, the fact that a mass resists changes in velocity is well established.
    The "why" however is not.

    As far as I know anyway....
  16. Jul 20, 2009 #15
    Momentum is best described as a mass's resistance to changes in velocity such as would result if a force were to act on it. It's a fundamental attribute of matter that you experience every time you're riding in a car and experience a hard, sharp turn. It's what makes your body want to keep going in a straight line along your previous path, but the car beneath you goes in a different direction, causing you to be pinned to the side of the car as it applies force to you, forcing you into a new path in a different direction. Is that real enough?

    --Mike from Shreveport
  17. Jul 20, 2009 #16
    Momentum of a moving object is something you try to stop it. The higher the momentum it has, the harder you have to try.
  18. Jul 22, 2009 #17
    It is probably pointless me giving my 2 cents but anyway.

    p = mv is a definition of mechanical momentum, but isn't really very fundamental at all.

    The canonical momentum which appears in Hamilton's equations is the mysterious one with all the links to quantum mechanics and generators of translations etc.
  19. Dec 19, 2009 #18
    I was trying to explain momentum to a friend and noticed that momentum is the derivative of energy in respect to velocity. Would this be a correct way to think about momentum, the rate of change of energy.
  20. Dec 19, 2009 #19


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    Hi user111_23! :wink:

    As pixel01 :smile: says …
    It is always conserved in collisions (while kinetic energy is not),

    so momentum measures the "oomph" available when something hits something else.

    Momentum is a quantity which an object has when it moves.

    It can transfer that quantity to another object.

    That quantity is never lost, it only moves from one object to another.

    It measures the ability to move another object …

    the more momentum you have, the more you can move something else …

    the less you have, the less you can move something else …

    if you do move something else, you must give up some of your own momentum.​
    It's like mgh, the gravitational potential energy …

    the "h" can be measured from any level (usually the most convenient one) … it's only the difference in h that matters.

    Electric potential (voltage) is potential energy per electric charge (V = PE/q), and it's only the difference in V that matters. :wink:
  21. Dec 19, 2009 #20


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    It is a little more complicated than that, but you are kind of discovering a key element of http://en.wikipedia.org/wiki/Lagrangian_mechanics" [Broken]. In Lagrangian mechanics the Lagrangian has units of energy and its derivative wrt the generalized velocity is the generalized momentum.
    Last edited by a moderator: May 4, 2017
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