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 10 kg, 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

Conserve momentum

m1 v1 + m2 v2 = m1 V1 + m2 V2

where v1, v2 are initial velocities and
V1 and V2 are final velocities.

Perfectly elastic means you also have
total kinetic energy before = total kinetic energy after