Question

A block of mass $M$ is placed on a rough floor of a lift. The coefficient of friction between the block and the floor is $\mu$. When the lift falls freely, the block is pulled horizontally on the floor. What will be the force of friction?

• A. $\mu Mg$
• B. $\frac{\mu Mg}{2}$
• C. $2\mu Mg$
• D. None of these

Answer 1

The correct option is D None of these

When the lift moves down ward with acceleration ${}^{\prime }{a}^{\prime }$ then effective acceleration due to gravity
$g^{\prime }=g-a$
$\therefore ,g^{\prime }=g-g=0$ [As the lift falls freely, so $a=g$]
So, force of friction $\mu m{g}^{\prime }=0$