Friction Notes
The force of friction Ff = uFN.
Ff is the force of friction.
FN is the normal force (force pushing up on the object to keep it from falling through the surface). On a
horizontal surface, this is equal to the force of gravity.
F = ma  Fg = mg = the weight of the object (in Newtons).
 is the coefficient of friction. It comes in two forms
 s = coefficient of static friction. Static means not changing, so this is for the case where an object is
not moving (yet).
 k = kinetic friction. This is for the case where the object is moving.
Note that the force of friction builds as a force is applied until it reaches a maximum, after which the
object starts moving. So
Ff <= s FN where Ff is at its maximum when the object is just ready to start moving.
On an inclined plane FN = mg cos (
On an inclined plane Ff = mg cos (

Note that for a horizontal surface, = 0 and mg cos(= mg.
This means that on an inclined plane the Ff will be lower than for the same object sitting flat on a
surface. This makes sense, since as you tip the plane the Ff diminishes until the object slides. Of
course, the Fg,x parallel to the plane is also increasing and is given by
Fg = mg sin(
Examples:
A 10Kg piece of wood slides on another piece of wood.
How much force can be applied before the piece slides?
Fn = 10kg * 9.8m/sec^2.
Ff =  FN = 0.42 * 98N = 41.16N
Once it starts moving, how much force is needed to keep it moving? Note that Ff remains constant once
it starts moving.
Ff = FN = 0.3 * 98N = 29.4N
If you are given the Normal force and Ff, or  and Ff, you can solve for the missing elements using
simple algebra.
Friction and motion.
With friction, an object keeps moving because there is a force applied.
Fnet = Fappl – Ff says that the sum of the forces acting on an object (Net force, total force) is equal to the
force being applied to the object minus the frictional force.
If there is no net force, the object is moving at constant velocity and Fappl = Ff.
If there is a net force,
Fnet = ma, where a is the net acceleration of the object and m is its mass. Find Fappl and Ff and subtract
them to find Fnet.