There are two types of friction, kinetic and static. Kinetic friction is the friction acting
on an object as it is moving, while static friction is the force acting on a still object. Wait,
you say, how can a force act upon a still object? Well, it's not really acting on it, but when
you try to move an object, the force of static friction will prevent you from doing so until the
force trying to push the object overcomes static friction.
Have you ever noticed how it is harder to get an object started than it is to keep it going.
That is because on any given surface, the coefficient of static friction is greater than the
coefficient of kinetic friction. These two coefficients are nothing but numbers; the higher
the number, the rougher the surface, and the lower the number, the smoother the surface. Thus
it is a valid statement to say that sandpaper has a higher coefficient of friction that silk.
The coefficients have no units, but are still very useful. Using them, we can find the forces
of friction acting upon the object. Here are the two equations that will help us do so:
FK = 
K * FN
FS = S * FN
The character m, or mu, represents the coefficients of friction. What follows the main
variable, as said in Knowing that the coefficient of static friction on this surface is 0.5, what is the force of
static friction on a 10kg block laying on a level plain?
This is where force diagrams come in.
FG = g * (5.98e24kg) * (10kg) / (6.37e6m)2
FG = 98.337N
Since, the block is laying on a flat surface, the normal force is equal to the force of
gravity.
Now for the force of friction.
FS = 0.5 * 98.337N
FS = 49.169N
It was not that imperative that we use a force diagram in this particular problem, but if
you use them now it will be used to using them later when you do need them. This lesson was
pretty straight forward, mainly because the normal force was equal to gravity.
In the next lesson, the object will be on a hill and the normal
force will be only part of gravity.
