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A block of mass 'm' 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

$\begin {array} {1 1} (1)\;Zero & \quad (2)\;2 \: \mu\: mg \\ (3)\;\large\frac{1}{2}\: \mu\: mg & \quad (4)\;\mu\: mg \end {array}$

(1) Zero