Find the acceleration of the system shown in the figure.

$A$ football of radius $R$ is kept on a hole of radius $r$ (where the diameter of the hole is $2r$ and $r < R$) made on a plank kept horizontally. One end of the plank is now lifted so that it gets tilted,making an angle $\theta$ with the horizontal as shown in the figure. The maximum value of $\theta$ such that the football does not start rolling down the plank satisfies (figure is schematic and not drawn to scale) -

$A$ body starts falling freely from height $H$ and hits an inclined plane in its path at height $h$ from the ground. As a result of this perfectly elastic impact,the direction of the velocity of the body becomes horizontal. The value of $\frac{H}{h}$ for which the body will take the maximum time to reach the ground is . . . . . . .

$A$ block of mass $m$ is placed on a smooth fixed inclined plane with an angle of inclination of $37^{\circ}$. $A$ horizontal force $F$ is applied to keep the block at rest. Find the value of $F$.

Three blocks of masses $2m$,$4m$,and $6m$ are placed as shown in the figure. If $\sin 37^{\circ}=\frac{3}{5}$ and $\sin 53^{\circ}=\frac{4}{5}$,the acceleration of the system is:

The acceleration of the system of two bodies over the wedge as shown in the figure is ....... $m/s^2$. (Take $g = 10 \ m/s^2$)