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Conservation of momentum, elastic collision problem? Help!

  1. Nov 18, 2012 #1
    Two titanium spheres approach each other head-on with the same speed and collide elastically After the collision, one of the spheres, whose mass is 300 g, remains at rest.

    What is the mass of the other sphere?

    What i did:

    m1v1 + m2v2 = m1u1 - m2u2
    v1 = 0 b/c at rest

    m2v2 = m1u1 - m2u2

    m2v2 = (m1 - m2)u

    The solution:

    v2 = 2u

    m2(2u) = (m1-m2)u
    2m2 = m1 - m2
    3m2 = m1
    m2 = m1/3

    What I do not understand:

    Why is v2 = 2u?? Is it because when m1 collides with m2, it transfers its speed to m2 so then m2 has twice its speed now? So then it's speed is the same as the initial speed times two?
    Last edited: Nov 18, 2012
  2. jcsd
  3. Nov 18, 2012 #2
    This is an elastic collision, so kinetic energy is conserved as well. There are two equations to solve, momentum and kinetic energy.
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