Kinetic Friction

When an object slides along a rough surface, the force of kinetic friction acts opposite to the direction of the object's velocity.

The magnitude of the force kinetic friction depends on the nature of the two sliding surfaces.

The friction force is approximately proportional to the normal force between the two surfaces, which is the force that either object exerts on the other, perpendicular to their common surface of contact.

We write the proportionality between the friction force F (fr) and the normal force F (N) as an equation by inserting a constant proportionality,