Question

A block of mass m is resting on the floor of a lift. The coefficient of friction between the block and the
floor is u. When the lift is falling freely, the limiting frictional force between block and surface is
umg
​

Answer 1

The limiting frictional force between the block and the surface is

${f_μ​}$ =μmg. . . . . .(1)

where, mg = normal force

When lift is free falling,  

Weight, W = mg = 0N, So there is no normal reaction between the surface of  block and the lift.

From equation (1),

${f_μ​}$ = 0N