A weight W rests on rough horizontal plane. If the angle of friction be $\mathrm{\theta }$, then calculate the least horizontal force that will move the body along the plane.

Two blocks (A) $2 \mathrm{kg}$ and (B) $5 \mathrm{kg}$ rest one over the other on a smooth horizontal plane. The coefficient of static and dynamic friction between (A) and (B) is the same and equal to $0.60$. The maximum horizontal force F that can be applied to (B) in order that both (A) and (B) do not have any relative motion is