Question

When does a force do work?

Answer 1

As we can see, this is purely logic based question. So, to find its answer we should first know about the force and work. After that we need to know the criteria when force will be said to do work and accordingly write the answer for the given question.

Complete answer:
We need to know about force and work. Already we know that a force is a push or pull acting on a body, which results in the interaction of that object with another object. Also, we know that work is defined as the force causing displacement.
Now we need to know the criteria when a force will be said to do work.
We can say that work is done on a body when
(A) Force is applied on the body
(B) There is a displacement in body
So, from the above discussion we conclude that work is said to be done by force only when the force is applied on a body and it makes the body to move i.e. displacement of the body is there.

Note: We know that work done = force $\times$ displacement. It means work is depending on both force and displacement. For example, if we are pushing a wall, in that case we are applying force but there is no movement in the wall. In this case no work is done. On the other hand, if we are pushing a wooden block and it is changing its position, in that case work is done.