Understanding Force

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Kinetic Friction | Dynamic Friction | Sliding Friction 


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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 Newton’s 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.

Kinetic friction can be demonstrated in the laboratory by the following experiment:

A wooden block placed on a wooden table is given a push to move it. It moves forward for a while, slows down and come to a halt. It slows down because of the kinetic friction that acted on its velocity, in the opposite direction to its motion, which eventually brings it to a stop.

To offset the existing kinetic friction between the wooden surfaces of both the block and the table, additional forward force is required. This is made possible by attaching a string to the block, and weights attached to the other end of the string. With the aid of a pulley system as shown below, the attached weights generate a forward force on the block making it to continue moving.


kinetic friction experiment 

By attaching the right weights on the string, the block will continue moving at constant velocity. At this point the net force acting on the block is zero.

The kinetic friction acting between the surfaces of two bodies in relative motion to each other is represented by the formula:


Fk  = µk R 

Or Fk  = µk mg

Or Fk  = µk N


Where Fk stands for kinetic friction, µk stands for coefficient of kinetic friction; R represents the force of reaction of the moving body on the surface of the second, which is equal to mg. This is also the same as the normal force N on the body.


See coefficient of kinetic friction here
See static friction here
See coefficient of static friction here
See friction here
See calculating friction here