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Motion cannot be created or destroyed

  1. Dec 7, 2008 #1
    Hello, my simple question I have is this:

    Am I right in thinking that the laws of physics actually say motion cannot be either created or destroyed.

    Beacause I looked at the laws of motion about six months ago, and they said that if you tried to stop something from moving, you will only start to moving yourself (and with the planet that you're standing on maybe). And there's no way around that.

    Am I wrong?
     
  2. jcsd
  3. Dec 7, 2008 #2

    Doc Al

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    Not sure what you mean by "motion".

    You're thinking of conservation of momentum. In any closed system, the total momentum remains constant.
     
  4. Dec 7, 2008 #3
    Motion is like when something moves, we say that it has 'motion' - it's moving
    that's all I meant.

    Alternatively you could say that we see that it's moving and it's going along at a speed of 100 miles per hour. It's definitely moving. The object has motion.

    Conservation of momentum just says that it's impossible to destroy that speed.
    You can only redistribute the 100mph with some neighbouring objects, but you can never really get rid of it entirely. All you're really doing is sharing it around.

    So the motion cannot be stopped. Is that true? Is it always true ?
     
  5. Dec 7, 2008 #4

    Doc Al

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    In that sense, "motion" is not conserved. Take two cars moving at 100 mph that crash head on. The final speed of both is zero. What happened to the "motion"?
    Conservation of momentum--not just "motion"--says that the total momentum of an isolated system remains constant. Momentum has a technical physics meaning. For a classical object of fixed mass, momentum = mass*velocity. (Note that velocity is a vector--direction counts.)
    No.
     
  6. Dec 7, 2008 #5
    Assuming the crash happenened on earth the motion went into the air molecules and vibrating metal molecules of the cars themselves. It turned into the motion of tiny molecles.

    That's my question. Is the motion always conserved. If you could add it all up (all the little wobbles of tiny molecules of metal and air and glass) would it still add up to the amount of motion that was in the car(s)?

    ps thanks for the replies. Yes I am aware that somebody discovered that the product of the mass times the speed stays the same, and that the product is called 'momentum'.

    pps I don't understand why it's neccesary to say it only applies to a closed system - because any object that gets hit by the moving stuff is by definition part of that system and the law would still obviously be true (if it ever was), on and on and on. Like with a row of dominoes...
     
  7. Dec 7, 2008 #6

    russ_watters

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    That sounds more like the definition of momentum, not motion. The difference is that momentum has a mass component. A bullet traveling at 1500 fpm hits a person and the person is knocked backwards at 100 fpm. The "motion" - the speeds - are vastly different. The momentum is conserved.
    I think the problem is that you are coming up with your own word to describe something that already has a word to describe it. Momentum is the concept you are looking for. Unless you really mean speed: in which case, no, speed is obviously not conserved.

    The word "motion" doesn't have a quantatative meaning in physics, so when you use it, you are almost certainly inadvertently substituting it for something else.
     
  8. Dec 7, 2008 #7

    Doc Al

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    Again, the term "motion" is very fuzzy. On the other hand, momentum is crisp and well-defined. In addition to momentum, one should consider mechanical energy and its transformation into internal energy (the random motion of the molecules).

    I would not say that "motion" is conserved. I would say that momentum is conserved and that any mechanical energy "lost" in the collision would have been transformed into another form of energy.
    How you define a system is arbitrary and depends on what you are interested in studying. With the two cars, if I take one car as my system then its momentum is not conserved since there was an external force exerted on it (from the other car). If I take both cars together as my system, then the total momentum starts at zero before the collision and remains zero after the collision. (The forces they exert on each other are now internal to the system.)
     
  9. Dec 7, 2008 #8
    russ, if every object in the universe had the same mass, we could easily use the word conservation of speed (well, velocity actually) instead of momentum, and physics would look a lot simpler. Especially to people like me.

    Unfortunately the objects of mass (atoms) clump together into larger masses (cars) - disguising the simplicity of the law.

    If all atoms weighed the same the law would be real simple
    If you add the speed of every atom you get the total momentum of the car.
    Or you can know how many atoms there are in the car and adding all their speeds gives you the total momentum.

    I'm not really commiting a cardinal sin by "confusing" speed with momentum am I.
     
    Last edited: Dec 7, 2008
  10. Dec 7, 2008 #9

    Doc Al

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    Sounds like you realize that conservation of "speed" just doesn't work, but for some reason you can't let it go. What's the point?
     
  11. Dec 7, 2008 #10

    russ_watters

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    True....
    No: when you start off with little knowledge of a subject, these kinds of misunderstandings happen all the time - that's kinda the whole point of learning!
     
  12. Dec 7, 2008 #11
    I was just trying to keep it simple that's all. Good point you made that direction is very important if the law is going to work at all. They probably realised this for the first time when the law of conservation of momentum was discovered. And the concept of velocity was born. - So I rediscovered tonight why we need to take direction into account. Cheers Doc ;-) (I'm such an idiot.)
     
  13. Dec 7, 2008 #12
    You are right in saying that in a system that is completely static, with no motion at all, nothing will move spontaneously... And motion will never be "created" in this system.

    A more correct rule is that momentum cannot be created or destroyed. The momentum that an object has is equal to its mass multiplied with its speed.

    i.e. If a ball weighs 2kg, and is travelling at 10m/s it's momentum would be 20.

    If this ball hits another ball that also weighs 2kg, and then they both start moving, then you can add the momentum of the two separate balls together, and it should theoretically equal 20. e.g. (After the collision) they are both travelling in the same direction with speeds of 3m/s and 7m/s:

    (3*2)+(7*2)= 20

    This is the same total momentum that the original ball had, and so momentum has not been created or destroyed.

    In the real world however, some of the momentum of the original ball would be converted into sound and heat energy in the air, and the momentum of the two balls would not be exactly the same as it was originally. However, this does not break the law of conservation of momentum because the extra momentum given to the air particles should equal the loss in momentum between the two balls.

    [edit]
    Have you also discovered that mass needs to be taken into account? Because even the most fundamental particles in the world have different masses. e.g. Electrons and Protons must have very different velocities for their momentum to be the same.
    [edit]

    Just a question for an expert: If kinetic energy is converted into an electro magnetic wave , then is the conservation of momentum broken? Or does light have a kind of "momentum".
     
    Last edited: Dec 7, 2008
  14. Dec 7, 2008 #13

    Dale

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    Yes, light has momentum.
     
  15. Dec 7, 2008 #14
    But momentum is mass*velocity... so does light have a mass? if not, then is it the definition of momentum that changes, or is it light that has a different "form" of momentum?
     
  16. Dec 7, 2008 #15
    Light has momentum equal to E/c.

    Problem with classic definition of momentum is that conservation of it is clearly not working when an object moves at a high speed. Usual definition for relativistic speed (= close enough to c) is p = m*gamma*v, where gamma is relativistic factor, 1/sqrt(1-(v/c)^2). Gamma goes to infinity as v approaches c, so there is no limit for the max momentum.

    As for the original topic - I have nothing to say that hasnt been already.
     
  17. Dec 7, 2008 #16
    Ahh, that makes a lot of sense, If the velocity is small, then 'gamma' will be very close to 1, and so the classical definition of momentum is still a good estimate.

    This definition still includes mass though, is there a single definition that will work for light and other massless objects?
     
  18. Dec 7, 2008 #17

    Dale

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    As I think you gathered from Zizy's comment, the equation p=mv is non-relativistic and at high speeds a more general form applies. Basically, momentum is a non-linear function of velocity for massive particles and massless "particles" like photons also carry momentum. I personally like the four-momentum formulation best because it captures the ideas of momentum, energy, and mass in a single convenient package that works the same way for massive and massless particles.
     
  19. Dec 7, 2008 #18
    In special relativity, E2 = (pc)2 + (mc2)2 holds for anything, massless or not. Massive particles satisfy p = γmv; massless particles satisfy p = E/c.
     
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