The typical surface of a solid is rough if not visibly then certainly on the microscopic level. It is no surprise then that when we bring two such surfaces in contact with one another they tend to lock into registry. To overcome this effect a force is required. There are two different situations to consider:
.
We perform experiments pulling bricks across the floor with a scale for the applied force. We note the following:
is generally larger than . That is once I get the brick going
the force required to maintain it in motion is smaller than the force
I needed to get it going.
nor depend on the area of the contact surface!
This perhaps surprising result was discovered by Leonardo da Vinci
by essentially the same experiment as the one we are doing.
and rise in proportion to the normal force
by which the two objects press against one-another.
and depend strongly on the nature of the
interface ie. on which surfaces are rubbing against one another.
To understand 2) and 3) it can be useful to note that according to these results it is the amount of force per area units available to drive the rough interfaces against each other which determines friction. If you wonder why racing cars have so broad tires it is primarily because this helps reduce the temperature of the interface which in turn is important for maintaining a large coefficient of friction (to be defined below).
Because of 2) and 3) we can characterize each interface by coefficients of
friction: the dimensionless constants of proportionality
between the friction and the normal force at the interface:
We measure these for one of our bricks by taking the ratio between the static and kinetic forces of friction and the weight of the brick.