Question

A boy is pushing hard on a wall. Is he doing work?
(A) No
(B) Can’t say
(C) May have done some work
(D) Yes

Answer 1

Hint: We will see that when a boy pushes a wall, he is applying force on the wall. We see that the wall shows no displacement. Work is given as the product of the forces and the displacement in the same direction. Hence the work done is zero.

Complete step by step answer:
Work done:
Work is said to be done when force is applied on an object and it gets displaced from its initial position. SI unit of work is Joules.
1 Joule is defined as 1 newton force is applied on an object and it gets displaced by 1 m distance.
$\Rightarrow$ $1joule = 1\,Newton\,metres \Rightarrow 1\,J = 1\,Nm$
Mathematically;
$\Rightarrow$ $W = F\,s\,\cos \theta $ … (1)
W = work done
F = applied force
s = distance covered by an object
$\theta $= angle between applied force and object displacement
In this case when boy pushes on the wall, it shows no displacement
S = 0 … (2)
Using equation (2) in (1), we get
$W = F.0 \Rightarrow W = 0$
This represents that no work is done when force is applied on a wall.
Secondly, the wall does not get displaced from its initial position. So,
S = 0

Thus, the correct option is A. He is not doing any work.

Note:
The boy still gets tired even when the total mechanical work is zero. This means that the boy is using energy. To clarify this apparent paradox lets understand and Note: that energy needn’t be only converted to work; it can also get converted to other types of energies like heat, light, sound etc. IN our case the boy gets tired trying to oppose the rigidity of the wall. Theoretically the boy is compressing the wall. However, the wall’s tensile strength is too high hence the negligible amount of compression that the boy would have caused goes unnoticed. The boy’s energy is also lost in the form of heat due to which we would practically see him sweating