You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 29 Next »

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:

Unknown macro: {latex}

\begin

Unknown macro: {large}

[ F_

Unknown macro: {k}

= \mu_

N]\end

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

ExamplesfromDynamicsinvolvingKineticFriction"> Examples from Dynamics involving Kinetic Friction

Error formatting macro: contentbylabel: com.atlassian.confluence.api.service.exceptions.BadRequestException: Could not parse cql : null

ExamplesfromEnergyandWorkinvolvingKineticFriction"> Examples from Energy and Work involving Kinetic Friction

Error formatting macro: contentbylabel: com.atlassian.confluence.api.service.exceptions.BadRequestException: Could not parse cql : null

ExamplesfromRotationandTorqueinolvingKineticFriction"> Examples from Rotation and Torque inolving Kinetic Friction

Error formatting macro: contentbylabel: com.atlassian.confluence.api.service.exceptions.BadRequestException: Could not parse cql : null

AllExampleProblemsinolvingKineticFriction"> All Example Problems inolving Kinetic Friction

Error formatting macro: contentbylabel: com.atlassian.confluence.api.service.exceptions.BadRequestException: Could not parse cql : null
  • No labels