{excerpt}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.
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h3. Static Friction as a Force
h4. The Limiting Size of Static Friction
The basic characteristics of static friction are well approximated by the limit expression:
{latex}\begin{large}\[ F_{s} \le \mu_{s} N\]\end{large}{latex}
where μ~s~ 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.
h4. 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.
{note}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".){note}
h3. 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.
{info}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|rigid body]). 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.{info}
h3. {toggle-cloak:id=examples} Example Problems involving Static Friction
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{contentbylabel:static_friction,example_problem|operator=AND|excerpt=true|showSpace=false|maxResults=50}
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