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Friction is a force whose tendency is to reduce the velocity of a body relative to its surroundings.

It is caused by interaction of the matter in the body with that of the environment in which it is traveling.

Friction is (at a fundamental level) an electromagnetic force.

That is, bits of the medium in or on which the body is traveling get in the way (on a molecular level) of bits of the body.

The less friction, the less the reduction in velocity.

If there were no friction, then motion would continue for ever.

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 $\mu_k$.

Linguistic Note

The word friction comes from the Latin word for rub.