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Applying the Law of Conservation of Momentum :S

  1. Jul 12, 2009 #1
    The law of conservation states that if two objects have a net force of zero, and if they were to collide with one another, the change in momentum of one object is equal in magnitude, but opposite in direction, to the change of momentum of the other object.

    Knowing this, what would happen if two vehicles, travelling at the same velocities (yet with different masses) collide with each other? Would the momentum be conserved? What would happen during this collision?

    Also, is it true that when an object with a heavier mass has a larger momentum than an object with a smaller mass?

    Thank you! (I am terribly poor at answering physics intuition questions :S)
  2. jcsd
  3. Jul 12, 2009 #2
    What you will study in physics is the momentum of rigid bodies. Unfortunately vehicles are not necessarily rigid bodies, they are made specifically to compress upon impact, in order to reduce the acceleration of the occupants in the vehicle(so that they have a better chance to survive). Momentum is conserved on the particle level, but this does not appear to be so on the macroscopic(vehicular level). Usually the two vehicles will hit, compress, and then come to rest, rather than be repelled or move together with a velocity

    Momentum is the product of Mass AND Velocity: p=mv

    Therefore, it may be true that a larger mass corresponds to a larger momentum than a smaller mass, but it may also be true that both masses have equal momentum, or the smaller mass has more momentum than the larger.
  4. Jul 12, 2009 #3
    hmmm, so, say, if both objects travelled at the same velocities, yet with different masses, before colliding, they would have the same momentums? My physics text book gave an example of equal momentum between two objects: A high-speed car would have the same momentum as a slow moving truck.

    And also, what if both vehicles travelled at different velocities before a collision; would both vehicles move back?

    Thank you so much for replying :)
  5. Jul 12, 2009 #4

    Doc Al

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    Staff: Mentor

    Momentum will be conserved at both levels.
  6. Jul 12, 2009 #5
    What i meant was if you look at the vehicles as if they were rigid bodies, they wouldnt APPEAR to conserve momentum like if we look at billiard balls which do appear to conserve like rigid bodies
  7. Jul 12, 2009 #6
    not quite. ill give you an example.
    you have two balls traveling towards each other. Ball 1 has a mass 10kg, and Ball 2mass 5kg

    They both move with velocity 2m/s

    the momentum of ball 1 is equal to (10)(2)= 20
    the momentum of ball 2 is equal to (5)(2)= 10

    so you have two different masses, moving with equal velocity, but having 2 different momentums.

    for objects with the same mass to have the same momentum, they must have the same velocity.

    the same goes with objects moving at the same velocity. to have equal momentum, they must have equal mass.

  8. Jul 12, 2009 #7

    Doc Al

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    Staff: Mentor

    Why wouldn't they? No need to think of them as rigid bodies; even if they crumple the total momentum is conserved. (Perhaps you're thinking of conservation of energy?)
  9. Jul 12, 2009 #8
    maybe my wording is just not great, because I was trying to convey what you said here.
  10. Jul 15, 2009 #9
    In a nutshell that would be the idea between elastic collisions versus inelastic collisions.
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