​ A body of mass 'm' dropped from a height 'h' reaches the ground with a speed of 0.8 . The value of workdone by the air-friction is :

Question

A body of mass 'm' dropped from a height 'h' reaches the ground with a speed of 0.8 \(\sqrt{gh}\) . The value of workdone by the air-friction is :

(1) –0.68 mgh

(2) mgh

(3) 1.64 mgh

(4) 0.64 mgh

Answer 1

 Correct Option is (1) –0.68 mgh

Work done = Change in kinetic energy

W_mg + W_air-friction = \(\frac12m(.8\sqrt{gh})^2-\frac12m(0)^2\)

W_{air- friction} = \(\cfrac{.64}2\)mgh - mgh = -0.68 mgh