Static friction

This article describes a force type.
View a complete list of force types

Definition

Intensional definition

Static friction is defined as the form of friction between two bodies, exerted via their surface of contact tangential to the surface of contact, when the surfaces of contact are not slipping against each other.

Note that this form of friction is acting not only when the two bodies are at rest with respect to each other, but also in the case of rolling -- when the bodies are moving relative to each other but there is no slipping at the region of contact.

Tendency-based definition

Static friction is defined as the form of friction between two bodies that opposes the tendency for the surfaces of contact to slip relative to each other. In magnitude, it is equal to the net external force between the two bodies tangential to the surface of contact, and in direction it is opposite to that net external force.

Model

According to the Coulomb model of friction, the maximum possible value of static friction is $\mu _{s}N$, where $N$ is the normal force between the two bodies and $\mu _{s}$ is the limiting coefficient of static friction. As long as the net external force parallel to the plane of contact has magnitude smaller than $\mu _{s}N$, there is no slippage and static friction acts. When the net external force exceeds this, slippage begins and static friction no longer applies.

Caveats

• Static friction acts when the surfaces in contact are at rest relative to each other. Thus, if one box is placed on to of the other and the lower box is dragged such that the upper box moves along with it, then the form of friction that is operating is static friction, not kinetic friction.
• Static friction opposes the net external force that would have produced relative slippage, and need not oppose the actual direction of motion.
• Static friction depends on the relative motion between the surfaces in contact, and not necessarily the bodies as a whole. Thus, it applies to the case of rolling but not to the case of pure rotation.

Energy

Static friction does not result in any conversion of mechanical energy into other forms of energy. This is in contrast with kinetic friction.

Related forces

Other forms of friction

• Kinetic friction or dynamic friction is the friction between two bodies that occurs via surfaces of contact that are slipping against each other.

Forces between rigid bodies

• Rolling friction: This is not really a form of friction, but rather, a normal force that effectively plays the role of friction due to local deformations of the surface.
• Normal force: The component of contact force in the direction perpendicular to the plane of contact.