Given, the mass of the body = m
The height from which the body dropped = h
The speed of the body when reached the ground,
vf=0.8√gh Initial velocity of the body, v = 0 m / s
Using the work-energy theorem,
Work done by gravity + Work done by air-friction = Final kinetic
energy - Initial kinetic energy
Wmg+Wair−friction=mvf2−mvi2 Here, work done by gravity = mgh
⇒
Wair−fricion=−mgh = 0.32 mgh - mgh = -0.68 mgh
The value of the work done by the air friction is -0.68 mgh.