is a material property, defined as the ratio of the Friction force (F) to the Normal (N) force. The friction force is the product of the coefficient of friction and the Normal force. The coefficient of friction can be static or kinetic, and varies according to the material properties; its value is determined experimentally.

Friction is the force that resists the (sliding) motion of two surfaces in contact. The force of friction is present everywhere two objects are in contact. It is easily observed when one attempts to displace a block sitting atop a table.

The Friction force is directed in the opposite direction of the Resultant Force acting on a body. In the case of kinetic friction, the direction of the friction force may or may not match the direction of motion: a block sliding atop a table with rectilinear motion is subject to friction directed along the line of motion; an automobile making a turn is subject to friction acting perpendicular to the line of motion (in which case it is said to be 'normal' to it). A motionless body is subject to static friction. The direction of the static friction force can be visualized as directly opposed to the force that would otherwise cause motion, were it not for the static friction preventing motion. In this case, the friction force exactly cancels the applied force, so the net force given by the vector sum, equals zero. It is important to note that in all cases, Newton's first law of motion holds.

Also see friction.