$A$ spring-block system is placed on a rough horizontal floor. The block is pulled towards the right to give the spring some elongation and then released.

$A$ block placed on a rough inclined plane of inclination $\theta = 30^{\circ}$ can just be pushed upwards by applying a force $F$ as shown. If the angle of inclination of the inclined plane is increased to $\theta = 60^{\circ}$,the same block can just be prevented from sliding down by application of a force of the same magnitude $F$. The coefficient of friction between the block and the inclined plane is:

Two blocks ($P$ and $Q$) with masses $2 \text{ kg}$ and $1.5 \text{ kg}$ respectively are joined by a massless thread. These blocks are mounted on a frictionless pulley which is fixed on the edge of a cube $(S)$, as shown in the figure. Block $P$ is positioned on the top surface which has no friction, and block $Q$ is in contact with the side surface, which has a coefficient of friction $\mu$. The cube $(S)$ moves towards the right with an acceleration of $g/2$, where $g$ is the gravitational acceleration. During this movement, the blocks $P$ and $Q$ remain stationary relative to the cube. The value of $\mu$ is . . . . . . . (Take $g = 10 \text{ m/s}^2$)

On which factors does the stopping distance of a vehicle depend?

Two balls of masses $m_1$ and $m_2$ are separated from each other by a powder charge placed between them. The whole system is at rest on the ground. Suddenly,the powder charge explodes and the masses are pushed apart. The mass $m_1$ travels a distance $s_1$ and stops. If the coefficients of friction between the balls and the ground are the same,the mass $m_2$ stops after travelling the distance:

Two blocks $A$ and $B$ of masses $5 \ kg$ and $10 \ kg$ respectively are placed as shown in the figure. The coefficient of friction between all the surfaces is $0.2$. $A$ force $F$ is applied on block $A$ at an angle of $30^{\circ}$ with the horizontal. If the system moves with constant acceleration,find the acceleration of block $B$. (Assume $g = 10 \ m/s^2$ and $F = 100 \ N$) (in $m/s^2$)