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 Force

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:

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 μk < μs.

Direction

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

Kinetic Friction as Non-Conservative Work

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, 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 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.

Example Problems involving Kinetic Friction

Examples from Dynamics involving Kinetic Friction

Examples from Energy and Work involving Kinetic Friction

Examples from Rotation and Torque inolving Kinetic Friction

All Example Problems inolving Kinetic Friction