When two bodies collide elastically,then

$A$ light particle moving horizontally with a speed $v_1$ strikes a very heavy block moving in the same direction with a speed $v_2$. The collision is elastic. After the collision,the velocity of the particle is:

$A$ ball after falling from a height of $10\, m$ strikes the roof of a lift which is descending down with a velocity of $1\, m/s$. The recoil velocity of the ball will be .............. $m/s$.

Derive the expressions for the velocities of two bodies after a one-dimensional elastic collision.

Two identical spheres $A$ and $B$ collide with two other identical spheres $C$ and $D$ with the same velocity $v$. What happens after the collision?

Assertion $(A)$: In an elastic collision of two billiard balls,both kinetic energy and linear momentum remain conserved.
Reason $(R)$: During the collision of the balls,as the collision is elastic,there is no exchange of energy. Therefore,both energy and momentum are conserved.