$A$ block of mass $5 \text{ kg}$ is in contact with a vertical wall as shown. Find the friction force acting on the block. (in $\text{ N}$)

Write the advantages and disadvantages of friction. Write the remedies to reduce friction.

Two identical balls are connected by a massless and inextensible thread of length $L$. The system is in gravity-free space with the thread just taut. At $t = 0$,each ball is imparted a velocity $v$; one ball is given a velocity towards the other ball,and the other is given a velocity perpendicular to the first. Then,

Which is a suitable method to decrease friction?

An ideal spring is connected between two blocks of masses $M$ and $m$. This system can move on a smooth horizontal table. The blocks are brought closer to compress the spring and then released. In the subsequent motion,select the true statement$(s)$:
$(a)$ They move in opposite directions with speeds inversely proportional to their masses.
$(b)$ The ratio of their speeds remains constant.
$(c)$ Linear momentum and energy of the system remain conserved.

$A$ sphere of mass $m$ and radius $R$ is kept on a rough horizontal surface and pulled with a horizontal force $F = \alpha t$,where $\alpha$ is a constant and $t$ is time. The coefficient of static friction is $\mu_s$ and the coefficient of kinetic friction is $\mu_k$. Which of the following graphs correctly shows the variation of the acceleration of the sphere with time?