A block of mass $4 kg$ rests on a horizontal table and a force of $10 N$ is applied to it in the horizontal direction. The coefficient of kinetic friction between the block and the table is $0.1$. Then the work done in $\sqrt{20}$ second by:    $(g = 9.8 m{s^{-}}^2)$

A block of mass $\mathrm{m}$ lying on a rough horizontal surface is acted upon by a horizontal force $P$ and another force $Q$ at an angle $\theta$ to the vertical line,The block will remain in equilibrium if the coefficient of friction between it and the surface is 

An insect crawls up a hemispherical surface very slowly, see Fig. 8.68. The coefficient of friction between the insect and the surface is $1/3$. If the line joining the centre of the hemispherical surface to the insect makes an angle a with the vertical, the maximum possible value of $\alpha$ is given by

Block $\mathrm{A}$  of mass is placed over the block  $\mathrm{B}$ of mass $8 kg$. The combination is placed over a rough horizontal surface, The coefficients of friction between $\mathrm{B}$ and the floor is $0.5$.The coefficient of friction between $\mathrm{A} \mathrm{and} \mathrm{B}$ is $0.4$ A horizontal force of $10N$ is applied on block B. The force of friction between$\mathrm{A} \mathrm{and} \mathrm{B}$ is: