Limiting Friciton

When two bodies that are not moving relative to each other are in contact, the friction force acting between their surfaces is known as static friction or limiting friction. This friction force will prevent one of the bodies from moving over the other, or sliding down a slopped surface, except an applied force is greater than it.

Static friction Fs, also known as limiting friction, is the maximum friction force that is produced and which must be overcome for a body to move over, or slide down the surface of another body.

Kinetic friction

Kinetic friction, also known as dynamic or sliding friction occurs when a body is in relative motion with another, with their surfaces in contact. It is the frictional force that slows down, and eventually stops a moving object.

For example, a soccer ball given a slight kick will begin to roll on the pitch, then slows down, and then come to a stop. The ball is slowed down, and then stopped after it has started moving with a velocity, as a result of the kinetic friction between its surface and the pitch.

Without confusing the two, the difference between static friction and kinetic friction is that static friction occurs when the object is stationary, that is, not moving, while kinetic friction takes place when the object has started moving. It is kinetic friction that makes sure an object does not continue to move forever when a force has acted on it as stated in Newtons first law of motion.

As is the case with static friction, kinetic friction depends on the nature of the materials or surfaces in contact, and the magnitude of the force that set the body in motion. However, kinetic friction is less than static friction for the same material surfaces in contact.