Question

Ball 2, with a mass of 30kg, is moving to the left at 10 m/s. with what velocity should ball 1, with a mass of 10kg, move to the right and collide with 2, so that ball 1 rebounds with a velocity of 30 m/s, and ball 2 with velocity of 10 m/s, after the collision? assume the collision to be perfectly elastic.

Answer 1

If the collision is elastic, then kinetic energy is conserved. Momentum will always be conserved.
\[m _{1}v _{1} + m _{2}v _{2} = m _{1}\prime v _{1}\prime + m _{2}\prime v _{2}\prime\]
is the only equation you will need. As you have all unknowns except v_1