Write the equation of mass energy equivalence.

$A$ ball strikes a smooth horizontal ground at an angle of $45^o$ with the vertical. What cannot be the possible angle of its velocity with the vertical after the collision ................. $^o$ (Assume $e \leq 1$ ).

$Assertion$ : In an elastic collision of two billiard balls, the total kinetic energy is conserved during the short time of oscillation of the balls (i.e., when they are in contact).
$Reason$ : Energy spent against friction does not follow the law of conservation of energy.

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 ............... $\mathrm{J}$

A simple pendulum, made of a string of length $l$ and a bob of mass $m$ , is released from a small angle $\theta_0$. It strikes a block of mass $M$, kept on a horizontal surface at its lowest point of oscillations, elastically. It bounces back and goes up to an angle $\theta_1$. Then $M$ is given by

A body of mass $m$ moving with a constant velocity $v$ hits another body of the same mass moving with the same velocity $v$ but in the opposite direction and sticks to it. The velocity of the compound body after collision is