Find minimum normal force to be applied by each hand to hold three identical books in vertical position. Each book has mass $m$ and the value of coefficient of friction between the books as well as between hand and the book is $\mu$.

A block of mass $m$ is kept on the floor of a freely falling lift. During the free fall of the lift, the block is pulled horizontally with a force of  $F=5 N, {\mu }_s=0.1$ .  The frictional force on the block will be

If in previous problem, an additional force  $F\text{'}=100 \mathrm{N}$ is applied in vertical direction as shown in figure. the friction force acting on the block is

A horizontal force of $25 \mathrm{N}$ is necessary to just hold a block stationary against a wall. The coefficient of friction between the block and the wall is  $0.4$. The weight of the block is