In an elastic collision of two billiard balls which of the following quantities is not conserved during the short time of collision

A particle $P$ moving with speed $v$  undergoes a head -on elastic collision with another particle $Q$ of identical mass but at rest. After the collision

The quantities remaining constant in a collision are

A body of mass $2\, kg$ moving with a velocity of $6\, m/s$ strikes inelastically another body of same mass at rest. The amount of heat evolved during collision is ............... $\mathrm{J}$

Explain the total linear momentum is conserved in an elastic collision and also explain the inelastic collision and completely elastic collision.

A point mass $M$ moving with a certain velocity collides with a stationary point mass $M / 2$. The collision is elastic and in one-dimension. Let the ratio of the final velocities of $M$ and $M / 2$ be $x$. The value of $x$ is