Sliding or kinetic friction is the drag force that results from two surfaces being pressed against each other while moving in relative motion. The sliding coefficient of friction is the dimensionless factor of this pressing force, resulting in the drag force for two specific surfaces.
In engineering, sliding friction occurs in numerous types of sliding components, such as journal bearings, cams, linkages and pistons in cylinders. Static friction is the friction required to move two surfaces that are not in relative motion.
The sliding coefficient of friction is normally much lower than the static coefficient of friction. This difference can be experienced when braking a car in slippery conditions. If the wheels lock up, the primary friction between the tires and the ground changes from static friction to sliding friction because the tires are no longer rolling with the ground speed, and the car will not decelerate as quickly.
Rolling friction occurs in components involving rolling motion such as ball bearings, roller bearings, wheels and linear bearings. Some components such as the gears involve a combination of both rolling and sliding friction.
When examined very precisely, however, all devices involving rolling friction technically involve sliding friction as well. This is because all rolling objects deform under contact pressure, even if only microscopically, resulting in sliding motion in addition to rolling motion.