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Conservation of momentum and energy problem

  1. Oct 27, 2014 #1
    Could someone please provide me with a elastic collision problem where there are two objects one object with an initial velocity=0 m/s and the other object with a final velocity= 0 m/s.

    Thank you
  2. jcsd
  3. Oct 27, 2014 #2

    Doc Al

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    Staff: Mentor

    How about this: A ball of mass m is resting on a frictionless table. A second ball, identical to the first, moves with speed v and collides dead on and elastically with the first ball. (All motion is along a single direction.)
  4. Oct 27, 2014 #3
    This is what I did, but I have assigned the values of the masses ,randomly, and the velocities based on the conservation of energy and something weird showed up.
    The Problem quantities that I assigned are the following :
    m1=2 kg
    initial velocity 1 = 2m/s
    final velocity 1 = 0 m/s
    m2= 4kg
    Initial velocity 2 = 0 m/s
    final velocity 2 = sqrt(2) m/s
    When I wanted to check if the energy and momentum is conserved, only the energy was conserved. How is that possible?
    should energy and momentum be conserved in elastic collisions or am I assigning wrong values?
  5. Oct 27, 2014 #4


    Staff: Mentor

    They have to have the same mass for the first mass to come to rest.
  6. Oct 27, 2014 #5
    So is it impossible for this to happen in nature if the objects have different mass?
  7. Oct 27, 2014 #6


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    Staff: Mentor

    In an elastic collision, yes.
  8. Oct 27, 2014 #7
    Thank you guys for the help
  9. Oct 27, 2014 #8

    Philip Wood

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    Gold Member

    You might think it's a bit strange that in this special collision the moving body, A, stops and the struck body, B, takes on its momentum and KE. But have you looked at it from the frame of reference moving at half the speed of A and in the same direction as A? In this frame, A and B are approaching each other with equal and opposite velocities, and bounce back after the collision with equal and opposite velocities. Nothing strange now!
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