* Frictional force opposes motion: Friction always acts in the opposite direction to the object's motion or intended motion.

* Work is force times displacement: The work done by a force is calculated as the force multiplied by the displacement of the object in the direction of the force.

* Opposite directions mean negative work: Since friction acts opposite to the direction of motion, the angle between the force and displacement is 180 degrees. The cosine of 180 degrees is -1. Therefore, the work done by friction is negative.

In simpler terms: Friction takes energy away from a moving object, causing it to slow down. Since energy is lost, the work done by friction is considered negative.