In loading a lorry a man lifts boxes each of weight 100 N through a height of 1.5 m. a) How much work does he do in lifting one box? b) How much energy is transferred when one box is lifted? c) If he lifts four boxes/minute at what power is he work

a) The work done by the man in lifting one box is given by the product of the force exerted by him and the displacement of the box lifted. Since the weight of the box is {eq}100 N {/eq} and it is lifted through a height of {eq}1.5 m {/eq}, the work done by the man is: {eq}begin{aligned} W &= Fd \ &= (100 N)(1.5 m) \ &= 150 J end{aligned} {/eq} Therefore, the man does {eq}150 J {/eq} of work in lifting one box. b) When the man lifts the box, he transfers potential energy to the box. The amount of energy transferred is equal to the work done by the man, which is {eq}150 J {/eq}. c) If the man lifts four boxes per minute, the total work done by him in a minute is: {eq}begin{aligned} W &= \text{ work done per box} times \text{ number of boxes lifted per minute} \ &= (150 J/box)(4 \text{ boxes/min}) \ &= 600 J/min end{aligned} {/eq} The power of the man is given by the rate at which he does work, which is the work done per unit time. Therefore, the power of the man is: {eq}begin{aligned} P &= \frac{\text{ work done}}{\text{ time taken}} \ &= \frac{600 J/min}{60 s/min} \ &= 10 \text{ W} end{aligned} {/eq} Therefore, the man is working at {eq}10 \text{ W} {/eq} when he lifts four boxes per minute.