Kinetic Friction

There are two types of kinetic friction based on the motion of the object. Examples of both these types can be observed in daily life.

Sliding Friction

In this type of friction, the object slides over the surface.

Examples

• A vehicle skidding on the road after applying brakes
• An ice skater or a skier coming to a stop
• A box being pushed on a floor
• Generation of heat due to rubbing of hands

Rolling Friction

In this type of friction, the object rolls over the surface

• A soccer ball coming to a stop after kicking it
• A golf ball coming to a stop after rolling over the green
• A roller skater coming to a stop
• A roller coaster slowing down before coming to a stop

Here are some laws that the kinetic friction follows.

• Acts in a direction opposite to the object’s motion
• Proportional to the normal force
• Independent of the area of contact between the surfaces
• Independent of the sliding speed as long as the speed is not too high

According to the laws of kinetic friction, the frictional force F_K is proportional to the normal force F_N. Therefore, the magnitude of kinetic friction is given by

F_K ∝ F_L

Or, F_K = μ_L F_L

Symbol of kinetic friction: F_K

Unit of kinetic friction: Newton or N

Dimension of kinetic friction: MLT^-2

The constant of proportionality μ_L is called the coefficient of kinetic friction. It is unitless and dimensionless. Its value depends upon the pair of surfaces. The coefficient of kinetic friction is less than that of static friction.

The work done by a force is the product of the force and displacement d. Therefore, the work done W by the kinetic friction force is

W = F_K x d

The kinetic friction remains constant over time as long as the normal load remains unchanged.

How to Find the Coefficient of Kinetic Friction

The coefficient of kinetic friction can be determined through experiments, such as measuring the force needed to overcome friction or measuring the angle at which an object will start to slide off an incline.