A body of mass $m$ dropped from a height $h$ reaches the ground with a speed of $0.8 \sqrt { g h }$. The value of work done by the air-friction is: (1) $- 0.68 m g h$ (2) $m g h$ (3) 0.64 mgh (4) 1.64 mgh
A body of mass $m$ dropped from a height $h$ reaches the ground with a speed of $0.8 \sqrt { g h }$. The value of work done by the air-friction is:\\
(1) $- 0.68 m g h$\\
(2) $m g h$\\
(3) 0.64 mgh\\
(4) 1.64 mgh