# Kinetic Friction

Written by Jerry Ratzlaff on . Posted in Kinematics

Kinetic friction, abbreviated as fk, also called dynamic friction or sliding friction, is the force opposing two objects rubbing together that are moving relative to each other. It is calculated using the normal force, which is the force perpendicular to the surface that the objects contact.

## Laws of Kinetic Friction

• The force of friction always acts in the opposite direction in which the body is moving between two surfaces in contact.
• The force of kinetic friction is independent of shape and area of the surfaces in contact.
• The magnitude of the kinetic friction bears a constant ratio to the normal reaction between the two surfaces.
• For moderate speeds, the force of friction remains constant.  But it decreases slightly with the increase in speed.

## Kinetic Friction formula

 $$\large{ f_k = \mu_k \; F_n }$$

### Where:

 Units English SI $$\large{ f_k }$$ = kinetic friction $$\large{ lbf }$$ $$\large{N}$$ $$\large{ \mu_k }$$ (Greek symbol mu) = kinetic friction coefficient $$\large{ dimensionless }$$ $$\large{ F_n }$$ = normal force $$\large{ lbf }$$ $$\large{N}$$