Kinetic Friction

This page defines and describes kinetic friction. Some of the information presented on this page is also present on the Friction page.

The Main Idea

Kinetic friction is a type of Friction that occurs between two touching objects that are moving with respect to each other at their point of contact. When two objects touch each other and there is sliding between their surfaces of contact, they exert a kinetic friction force on each other. The kinetic friction force acting on each object acts in a direction opposite to its direction of motion relative to the other object. If the kinetic friction force is the only force acting on each object, they will eventually come to rest relative to each other, at which point the friction force between them becomes static. For example, consider a crate sliding across the floor. It will slow down and eventually come to rest due to the kinetic friction force between it and the floor, unless another force acts on it, such as the pushing of a person that wants to keep the crate moving. Kinetic friction force within a system lowers the system's kinetic energy, converting it to Thermal Energy. In other words, sliding between surfaces causes the surfaces to heat up. Usually in mechanical physics, this thermal energy is no longer considered part of the system and is considered lost because it usually dissipates into the environment without ever being converted back into mechanical energy. This means kinetic friction does negative work on the system. In addition to dissipating energy as thermal energy, kinetic friction may also dissipate a smaller amount of energy as sound, and can wear down surfaces if sufficiently intense.

A Mathematical Model

The magnitude of the kinetic friction force between objects is given by

[math]\displaystyle{ F_k = \mu_k * N }[/math]

where [math]\displaystyle{ \mu_k }[/math] is the coefficient of kinetic friction of the objects and [math]\displaystyle{ N }[/math] is the normal force between the objects. [math]\displaystyle{ \mu_k }[/math] is a property of the materials of the surfaces in contact and is usually less than 1. [math]\displaystyle{ \mu_k }[/math] has no units because it is a ratio of one force to another. For any given pair of surfaces, their [math]\displaystyle{ \mu_s }[/math] value is greater than their [math]\displaystyle{ \mu_k }[/math] value.

Note that this formula indicates that the magnitude of the kinetic friction force does not depend on the speed of the sliding between the objects.

A Computational Model

This is a VPython simulation of a box sliding along a surface. At first it experiences no friction, but when it passes the red line, it experiences a constant kinetic friction due to the floor until it comes to rest.

Kinetic friction simulation

Click "View this program" on the top left corner to view the VPython code.

See also

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