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{excerpt}The specific manifestation of [friction] that is directly opposed to an object's sliding motion along a surface.  The force of kinetic friction has a size independent of the speed of the object, and proportional to the [normal force] exerted on the object by the surface.{excerpt}

h3. Kinetic Friction as a Force

h4. Magnitude

For an object that is already sliding along a surface or is accelerating from rest on a surface, the size of the friction force will be proportional to the [normal force] exerted by the surface on the object.  This mathematical relationship is usually stated:

{latex}\begin{large}\[ F_{k} = \mu_{k} N\]\end{large}{latex}

where μ~k~ is a constant of proportionalty called the coefficient of kinetic friction.  The coefficient of kinetic 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.

The coefficient of kinetic friction for a given object on a given surface will usually be *different* than the corresponding coefficient of [static friction].  It is usually the case that &mu;~k~ < &mu;~s~.  

h4. Direction

Kinetic friction is always directed opposite to the direction of the [velocity].  

h3. Kinetic Friction as Non-Conservative Work

h4. When the Surface is at Rest

When an object is sliding on a surface that can be considered to be at rest in an [inertial frame of reference|inertial reference frame], kinetic friction is the prototypical [non-conservative force].  When the motion of an object sliding on a surface is viewed from a frame at rest with respect to the surface, the force of friction always opposes the object's motion, and so always does negative [work].  For the special case of a friction force with _constant magnitude_ , the definition of [work] can be integrated to give {_}W{~}f{~}{_} = -- {_}F{~}k{~}d{_} where _d_ is the [distance] traveled by the object along the surface.

h4. When the Surface is Moving

Finding the [work] done by [friction] can be confusing when a reference frame is chosen in which the surface is _moving_.  See the discussion of [static friction] for more details.

h3. Example Problems involving Kinetic Friction

h4. {toggle-cloak:id=dyn} Examples from Dynamics involving Kinetic Friction

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h4. {toggle-cloak:id=nc} Examples from Energy and Work involving Kinetic Friction

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h4. {toggle-cloak:id=rot} Examples from Rotation and Torque inolving Kinetic Friction

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h4. {toggle-cloak:id=all} All Example Problems inolving Kinetic Friction

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