Question

A block of mass ${}^{\prime }{m}^{\prime }$ is placed on floor of a lift which is rough. The coefficient of friction between the block and the floor is $\mu$. When the lift falls freely, the block is pulled horizontally on the lift floor. The force of friction is:

• A. zero
• B. $2\mu mg$
• C. $\frac{1}{2}\mu mg$
• D. $\mu mg$

Answer 1

The correct option is A zero

As the system falls freely, the acceleration of entire system will be $g$. Hence, the acceleration of $m$ will also be $g$. Therefore, only force acting on the block will be that of gravity. This means that the normal force on it will be zero. Since friction cannot exist without normal force, friction force on the block will be zero.