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If an object is at rest with respect to a surface, friction will attempt to resist efforts to start the object sliding along the surface. Friction has the goal of keeping the object static _with respect to the surface_. This [static friction] is a response force -- it provides just enough force to keep the object stationary, and no more. [Static friction|static friction] is characterized by a limiting value. When the net force attempting to create sliding motion exceeds a certain value, [static friction] will be unable to prevent motion.
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h3. Quantitative Model of Static Friction
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}
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