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

$A$ small steel ball bounces on a steel plate held horizontally. On each bounce,the speed of the ball arriving at the plate is reduced by a factor $e$ (coefficient of restitution) in the rebound,so that $V_{\text{upward}} = e V_{\text{downward}}$. If the ball is initially dropped from a height of $0.4 \text{ m}$ above the plate and if $10 \text{ s}$ later the bouncing ceases,the value of $e$ is:

$A$ wooden block of mass $m$ moves with velocity $v$ and collides with another block of mass $4m$,which is at rest. After the collision,the block of mass $m$ comes to rest. The coefficient of restitution will be:

$A$ small ball falling vertically downward with a constant velocity of $4 \ m/s$ strikes elastically a massive inclined cart moving with a velocity of $4 \ m/s$ horizontally,as shown in the figure. The velocity of the rebound of the ball is

$A$ ball is allowed to fall freely from a height of $42 \ m$ from the ground. If the coefficient of restitution between the ball and the ground is $0.4$,then the total distance travelled by the ball before it comes to rest is (in $m$)

$A$ ball is moving with speed $20 \, m/s$ collides with a smooth surface as shown in the figure. The magnitude of the change in velocity of the ball will be ........... $m/s$.