Here's why:

* Work is defined as the force applied multiplied by the displacement in the direction of the force.

* Displacement is the change in position of an object.

* If the force is perpendicular to the motion, there is no displacement in the direction of the force.

Example:

Imagine pushing a box horizontally across a floor. If you push directly sideways (perpendicular to the motion), you might be exerting force, but you're not doing any work on the box in terms of moving it forward. The box will only move forward if you apply force in the direction of the motion.

Key Takeaway: Work is done only when a force causes an object to move in the direction of the force.