Friction
It is usually found experimentally that the force required to slide one
object over another is proportional to the normal force pressing the surfaces
together as expressed by 
where 
is called the “coefficient
of friction” and represents the roughness of the surfaces in contact.
In order to express the reality that it is harder to make an object start
moving than to keep the object moving, we can discuss the coefficient of static
(not moving) friction 
(to get it moving)
versus the coefficient of kinetic (in motion) friction 
(to keep it
moving). In general,
.