Question to find air resistance

1. Apr 21, 2007

siddharthmishra19

1. The problem statement, all variables and given/known data

A body of mass m is dropped from a certain height without initial velocity. At the time t when its speed is v, what is the work done by the air resistance to retard the body?

2. Relevant equations

Laws of conservation of momentum and inertia

3. The attempt at a solution

$$E_(final) - E_(initial) = W_net$$

$$mv^2/2 - mgh = Fs = Fvt$$

Solving,

$$F = (mv^2/2 - mgh)vt$$

Something is definitely wrong... the answer is given as:

$$F = 0.5mv(gt-v)$$

2. Apr 21, 2007

SGT

s would be vt if the body moved with constant velocity v during the time t.
The work done by the air resistance is Fh, where F is the mean value of the air resistance.