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 $N$ be the normal reaction of $S$ on $B$.
Then the coefficient of static friction is defined and denoted:
- $\mu_s = \dfrac F N$
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 $\mu_k$.
The word friction comes from the Latin word for rub.