Power needed to lift a box?

Question

If a man has an average useful power output of 40.0 W, what minimum time would it take him to lift thirty 14-kg boxes to a height of 1.2 m? The value of g is 9.80 m/s^2.

Answer 1

The mechanical power equals P = W / t, where W is the work required to lift the boxes. This work equals the change in their potential energy:
W = ΔEP = mgh = 30 x 14 kg x 9.8 m/s2 x 1.2 m = 4939.2 J.
We solve for t:
t = W / P = 4939.2 J / 40.0 W = 123 s = 2 minutes