Definition:Friction
Definition
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.
Coefficient of Friction
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.
Also see
- Results about friction can be found here.
Sources
- 1966: Isaac Asimov: Understanding Physics ... (previous) ... (next): $\text {I}$: Motion, Sound and Heat: Chapter $3$: The Laws of Motion: Inertia
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): friction
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): friction
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): friction
- 2021: Richard Earl and James Nicholson: The Concise Oxford Dictionary of Mathematics (6th ed.) ... (previous) ... (next): friction (frictional force)