The centre of mass of an extended body on the surface of the earth and its centre of gravity

A $$1 \mathrm{~kg}$$ ball moving at $$12 \mathrm{~ms}^{-1}$$ collides with a $$2 \mathrm{~kg}$$ ball moving in opposite direction at $$24 \mathrm{~ms}^{-1}$$. If the coefficient of restitution is $$2 / 3$$, then their velocities after the collision are

A ball hits the floor and rebounds after an inelastic collision. In this case

During inelastic collision between two objects, which of the following quantity always remains conserved?