A ball of mass 100 g is moving with a velocity of 3 m/s towards east. A cube of mass 200 gram is moving towards west with a velocity of 7 m/s. If the collision is perfectly elastic, what is the final velocity of the cube in m/s?

Collision is short-duration interaction between two bodies or more than two bodies simultaneously causing change in motion of bodies involved due to internal forces acted between them during this. Collisions involve forces (there is a change in velocity).