Definition:Friction/Coefficient

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Definition

Let $B$ be a body at rest on a plane surface $S$ on which friction acts.

Let $N$ be the normal reaction of $S$ on $B$.

Let a force be applied to $B$ parallel to $S$.


Coefficient of Static Friction

Let $F$ be the magnitude of that force in the limiting case when $B$ is just about to move.

Then the coefficient of static friction is defined and denoted:

$\mu_s = \dfrac F N$


Coefficient of Kinetic Friction

Let $F$ be the magnitude of that force needed to keep $B$ moving at a constant velocity.

Then the coefficient of kinetic friction is defined and denoted:

$\mu_k = \dfrac F N$


These coefficients of friction depend upon the materials out of which $B$ and $S$ are made.

It is usual for $\mu_s$ to be greater than $\mu_k$.


It is noted that the coefficients of friction is independent of the area of contact between the two materials.


Also see

  • Results about coefficients of friction can be found here.


Linguistic Note

The word friction comes from the Latin word for rub.


Sources

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