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

Here's why:

* Work is defined as the product of force and displacement in the direction of the force.

* Cosine of 90 degrees is zero.

Therefore, when the angle is 90 degrees, the cosine term becomes zero, making the entire work done equal to zero.

Example:

Imagine pushing a box horizontally across a floor. If you push straight down on the box (force perpendicular to motion), you are doing no work on the box. The box doesn't move in the direction of your force.

Let me know if you'd like to explore more about work and its relationship with force and displacement!