A ball of mass m1, moving with speed v, collides with a stationary ball of mass m2. The first ball rebounds with speed 1/5 v, whereas the other moves off with speed 4/5 v in the direction of the original velocity of the first ball.

1. Use the conservation of momentum to show that (m1/m2) = 2/3

Set up an equation that equates the initial momentum with the final momentum:


m_{\small 1} v = m_{\small 1} \frac {-v} 5 + m_{\small 2} \frac {4 v} 5


Note the negative sign - that's because m1 is moving in the negative directon after the collision. Now solve for m1/m2.

Thanks a bunch ebaines :)