The specific manifestation of friction which attempts to resist efforts to move an object that is currently at rest with respect to a surface. If possible, static friction provides just enough force to keep the object stationary, and no more. When the net force attempting to create sliding motion exceeds a certain limiting value proportional to the normal force exerted by the surface on the object, static friction will be unable to prevent motion.

Static Friction as a Force

The Limiting Size of Static Friction

The basic characteristics of static friction are well approximated by the limit expression:

where μs is the coefficient of static friction. The coefficient of static friction is a dimensionless number, usually less than 1.0 (but not required to be less than 1.0). Rough or sticky surfaces will yield larger coefficients of friction than smooth surfaces. N is the normal force exerted on the object by the surface which is creating the friction, which is a measure of the strength of the contact between the object and the surface.

Determining the Force of Static Friction

To determine the force of static friction on an object, calculate the net force in the absence of any friction and compare it to the limiting value of the friction force. If the maximum static friction force is larger than the net force in the absence of friction, then friction will provide the force necessary to make the total net force equal zero assuming that the net force has no component perpendicular to the surface. If, however, the maximum static friction force is less than the net force in the absence of friction, static friction will not apply (it will not provide a force). Instead, kinetic friction will apply.

It is very important to remember that for an object at rest on a surface and subject to no forces that would act to cause sliding, the static friction force will be zero! (The object will not move without friction, so friction "has no job to do".)

Static Friction as Non-Conservative Work

The work done by static friction would seem to be obviously zero, since an object subject to static friction is required to be stationary. There is an important loophole, however, in the fact that the object is only required to be stationary with respect to the surface in order for static friction to apply. Thus, if the surface itself is moving, the object is moving as well! This loophole is relevant in problems where one object sits on another, e.g. a box on a truck bed or a block on a second block, etc. In these examples, static friction can be the sole force responsible for changing the horizontal motion of the box, the top block, etc. Thus, it must be the case that the friction can do work.

There is at least one other major loophole that allows static friction to do work. Real objects like people and cars are actually deformable (they are not perfectly modeled as rigid bodies). Thus, a person can cause their center of mass to translate without moving their feet, and a car can move forward without translating the point of contact between the tires and the ground. In this way (when viewing the person or car as a single object) the force of static friction on the person's feet or the car's tires can be made to do work.

Example Problems involving Static Friction