A box of mass $8 \mathrm{kg}$ is placed on a rough inclined plane of inclination $\theta$. It's downward motion can be prevented by applying an upward pull $F$ and it can be made to slide upwards by applying a force $2F$. The coefficient of friction between the box and the inclined plane is

A block of mass $15 \mathrm{kg}$ is resting on a rough inclined plane as shown in the figure. The block is tied by a horizontal string which has a tension of $50 \mathrm{N}$. The coefficient of friction between the surfaces of contact is

Two blocks of masses $0.2 \mathrm{kg}$ and $0.5 \mathrm{kg}$, which are placed $22 \mathrm{m}$ apart on a rough horizontal surface $(\mu =0.5)$, are acted upon by two forces of magnitude $3 \mathrm{N}$ each as shown in figure at time $t=0$. Then, the time $t$ at which they collide with each other is