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

$A$ 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:

The sphere $A$ of mass $m$ moving with a constant velocity hits another sphere $B$ of mass $2m$ at rest. If the coefficient of restitution is $0.4$,the ratio of the velocities of the spheres $A$ and $B$ after collision is

$A$ ball is dropped from a height of $5\,m$. The time after which the ball stops rebounding,if the coefficient of restitution between the ball and the ground is $e = 1/2$,is ............. $s$.

$A$ sphere collides with another sphere of identical mass. After collision,the two spheres move. The collision is inelastic. Then the angle between the directions of the two spheres is

Two pendulums with identical bobs and lengths are suspended from a common support such that in rest position the two bobs are in contact (figure). One of the bobs is released after being displaced by $10^{\circ}$ so that it collides elastically head-on with the other bob.
$(a)$ Describe the motion of two bobs.
$(b)$ Draw a graph showing variation in energy of either pendulum with time,for $0 \leqslant t \leqslant 2T$,where $T$ is the period of each pendulum.