4.1 Sliding friction
Sliding friction force opposes the movement between two contact surfaces that slide against each other. This force depends on the type of contact surfaces (materials and finishing level) and on the load applied in the direction perpendicular to the motion direction (Normal force).
In mathematical terms, the sliding friction force is defined as follows:
where: br = sliding friction coefficient N = normal force (or load)
If two bodies are initially stationary, the resistance force is called the static friction force and represents the minimum force that must be applied to start moving the two bodies.
When the two bodies are in relative motion, a force lower than the static friction force is sufficient to keep the speed constant: this is called the dynamic friction force. The friction coefficient is obtained experimentally for both static friction and dynamic friction.