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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. |
Kinetic Friction as a ForceMagnitudeFor 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 |
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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 |
where μk 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 μ~k~ < μ~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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{contentbylabel:that μk < μs. DirectionKinetic friction is always directed opposite to the direction of the velocity. Kinetic Friction as Non-Conservative WorkWhen the Surface is at RestWhen an object is sliding on a surface that can be considered to be at rest in an inertial frame of reference, 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 Wf = – Fkd where d is the distance traveled by the object along the surface. When the Surface is MovingFinding 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. Example Problems involving Kinetic Friction Examples from Dynamics involving Kinetic Friction| Content by Label |
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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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