Question

Suppose a man is trying to push a wall. But the wall doesn't move. What is the amount of work done? Calculate the amount of work done in lifting a body of mass 3 kg through a distance of 11 metres.

Answer 1

When we push hard against the wall, the force we exert on the wall does no work, because there is no displacement in the wall. However, in this process, our muscles are contracting and relaxing alternately and internal energy is being used up, That is why we do get tired.

As, we know that :-

$$\begin{gathered} \mathrm{work}\mathrm{done}=\mathrm{force}\times \mathrm{displacement}=\mathrm{F}\times \mathrm{s} \\ \mathrm{here},\mathrm{F}=\mathrm{mg}=3\mathrm{kg}\times 9.81\mathrm{m}/{\mathrm{s}}^2=29.43\mathrm{N} \\ \mathrm{s}=11\mathrm{m} \\ \mathrm{thus},\mathrm{W}=29.43\times 11 \\ \mathrm{so},\mathrm{work}\mathrm{done}\mathrm{W}=323.73\mathrm{J} \end{gathered}$$