Kinetic friction: Difference between revisions

Revision as of 15:45, 22 November 2009

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Definition

Intensional definition

Kinetic friction, or dynamic friction, is defined as the form of friction between two bodies via their surface of contact (i.e., the force acting parallel to the plane of contact) when the two surfaces of contact are slipping against each other.

Tendency-based definition

Kinetic friction is the form of friction that opposes the slipping against each other of two surfaces in contact. Its direction is opposite the direction of relative motion of the surfaces in contact. Note that it need not oppose the net external force between the surfaces in contact.

Model

The typical model used for kinetic friction, which is applicable for small relative speeds, is the Coulomb model of friction, which states that the magnitude of kinetic friction is $\mu _{k}N$ , where $\mu _{k}$ is the coefficient of kinetic friction (dependent on the nature of the surfaces in contact) and $N$ is the normal force between the bodies (measuring how hard they are pressed against each other).

Caveats

Kinetic friction depends on whether the surfaces are slipping relative to each other. Thus, even if both surfaces are moving, as long as they are moving together, there is no kinetic friction. An example might be one box placed on top of another box that is being dragged along. If the two boxes move together, there is no kinetic friction between them.
Kinetic friction operates when the surfaces are slipping against each other, and opposes the direction of slippage of the surfaces, rather than the direction of relative motion of the bodies. Thus, in the case of rolling, there is no kinetic friction between the bodies. In other cases of mixed translation-cum-rotation, the direction in which the surfaces are slipping against each other depends on the contributions of both the translation and rotation components of the relative motion. For instance, if a wheel in contact with the ground is undergoing pure rotation, it will encounter kinetic friction from the ground opposing the direction of rotation.
For small speeds, the magnitude of kinetic friction is independent of the magnitude of the speed.
Kinetic friction may either support or oppose other external forces that act parallel to the plane of contact. For instance, a body sliding downhill along an inclined plane experiences kinetic friction in the uphill direction and a component of gravitational force in the downhill direction. A body sliding uphill, on the other hand, experiences both kinetic friction and a component of gravitational force in the downhill direction.

Related forces

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 ==Model==
-The typical model used for kinetic friction, which is applicable for small velocities, is the [[Coulomb model of friction]], which states that the magnitude of kinetic friction is <math>\mu_kN</math>, where <math>\mu_k</math> is the [[coefficient of kinetic friction]] (dependent on the nature of the surfaces in contact) and <math>N</math> is the [[normal force]] between the bodies (measuring how hard they are pressed against each other).
+The typical model used for kinetic friction, which is applicable for small relative speeds, is the [[Coulomb model of friction]], which states that the magnitude of kinetic friction is <math>\mu_kN</math>, where <math>\mu_k</math> is the [[coefficient of kinetic friction]] (dependent on the nature of the surfaces in contact) and <math>N</math> is the [[normal force]] between the bodies (measuring how hard they are pressed against each other).
+==Caveats==
+* Kinetic friction depends on whether the surfaces are slipping ''relative'' to each other. Thus, even if both surfaces are moving, as long as they are moving together, there is no kinetic friction. An example might be one box placed on top of another box that is being dragged along. If the two boxes move together, there is no kinetic friction between them.
+* Kinetic friction operates when the ''surfaces'' are slipping against each other, and opposes the direction of slippage of the surfaces, rather than the direction of relative motion of the bodies. Thus, in the case of [[rolling]], there is no kinetic friction between the bodies. In other cases of mixed translation-cum-rotation, the direction in which the surfaces are slipping against each other depends on the contributions of both the translation and rotation components of the relative motion. For instance, if a wheel in contact with the ground is undergoing pure rotation, it will encounter kinetic friction from the ground opposing the direction of rotation.
+* For small speeds, the magnitude of kinetic friction is independent of the magnitude of the speed.
+* Kinetic friction may either support or oppose other external forces that act parallel to the plane of contact. For instance, a body sliding downhill along an inclined plane experiences kinetic friction in the uphill direction and a component of gravitational force in the downhill direction. A body sliding uphill, on the other hand, experiences both kinetic friction and a component of gravitational force in the downhill direction.
 ==Related forces==