Two masses of $10 \,kg$ and $20 \,kg$ respectivety are connected by a massless spring as shown in fig. A force of $200 \,N$ acts on the $20 \,kg$ mass At the instant shown the $10 \,kg$ mass has acceleration $12 \,m / s ^2$ towards right. The acceleration of $20 \,kg$ mass at this instant is ........ $m / s ^2$

Block $A$ of mass $4 \;kg$ is to be kept at rest against a smooth vertical wall by applying a force $F$ as shown in figure. The force required is .......... $N$ $\left(g=10 m / s ^2\right)$

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

If net force on a system is zero then

The horizontal acceleration that should be given to a smooth inclined plane of angle $sin^{-1}\, (1/l)$ to keep an object stationary on the plane relative to the inclined plane is