Question 11: In loading a truck, a man lifts boxes of 100 N each through a height of 1.5 m.
(i) How much work does he do in lifting one box?
(ii) How much energy is transferred when one box is lifted?
(iii) If the man lifts 4 boxes per minute, at what power is he working? (g = 10 m s^-2)

Answer: (a) Here the work is done against the force of gravity. F = Weight of the box

Work done to lift the one box up to a height of 1.5 m = $F\times s=100\mathrm{N}\times 1.5\mathrm{m}=150\mathrm{J}$

(b) The work done by the man against the force of gravity is stored in the box in the form of its potential energy. Hence, the energy transferred to lift a box is equal to the work done. 

Energy transferred in lifting one box = 150 J

(c) Work done by the man in lifting one box = 150 J

Work done by the man in lifting 4 boxes = 150 J$\times$4 = 600 J

Time taken by the man to lift the 4 boxes = 1 minute = 60 seconds

Power = $\frac{W}{t}=\frac{600\mathrm{J}}{60\mathrm{s}}=10\mathrm{W}$