A secret agent is locked in a room. He pushes against the door but cannot open it. Finally, he falls to the floor exhausted. Has he done any work in the door? Why or why not?

Answer: There is not work done at the door because the door did not move.

Explanation: Work is defined as the movement done by a force.

So if you move to apply a force F in an object and you move it a distance D, the work applied on the object is

W = F*D

In this case, the secret agent pushes against the door, so there is a force, but the agent does not move the door, so D = 0, so there is no motion of the door, which implies that there is no work done at the door.