The work done by a force is zero when the angle between the force and displacement is 90 degrees.

Here's why:

* Work = Force x Displacement x cos(theta), where theta is the angle between the force and displacement.

* When theta is 90 degrees, cos(90) = 0.

* Therefore, the entire work equation becomes zero.

In simpler terms:

* When a force acts perpendicular to the direction of motion, it does no work. Imagine pushing a wall: you're applying force, but the wall doesn't move, so no work is done.