Here's why:

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

* Mathematically: Work (W) = Force (F) * Displacement (d) * cos(theta)

* Where theta is the angle between the force and displacement.

* When the force is perpendicular to the displacement, the angle between them is 90 degrees.

* The cosine of 90 degrees is 0.

* Therefore, the work done is: W = F * d * 0 = 0

Examples:

* Pushing a wall: You exert a force on the wall, but it doesn't move. Since there's no displacement, no work is done.

* Carrying a heavy box horizontally: You exert an upward force to counter gravity, but the box moves horizontally. The force is perpendicular to the displacement, so no work is done by the upward force.

* A satellite orbiting the Earth: The gravitational force acts towards the Earth's center, but the satellite's velocity is perpendicular to the gravitational force. Therefore, the gravitational force does no work on the satellite, and its speed remains constant.

Important Note: While the force itself might not do work, other forces might be acting on the object simultaneously. For example, in the case of carrying a box, even though you are not doing work, the force of gravity is doing negative work as the box moves downward.