The Force of Friction

Friction is a force that is created whenever two surfaces move or try to move across each other.

Friction always opposes the motion or attempted motion of one surface across another surface.
Friction is dependant on the texture of both surfaces.
Friction is also dependant on the amount of contact force pushing the two surfaces together (normal force).

There are two different kinds of contact friction

Static Friction (f_s): The force of friction present when there is no relative motion (no sliding) between the two surfaces in contact. As you will see in the simulation below, static friction has a range from zero up to some maximum value f_{s max}.
Kinetic Friction (f_k): The force of friction present when there is relative motion (sliding) between the two surfaces in contact.

In this simulation you see a block sitting on a level table. You can place an applied force on the object by pressing the "More Force" button. Each time you press the button the applied force will increase. As you use this simulation there are several things you should notice. [QuickTime version]

On a level surface, the normal force (F_N) is always equal and opposite to the weight (only on a level surface).
The force of static friction (f_s) cancels out the applied force right up to and including when static friction reaches its maximum (f_{s_max}).
For applied forces greater than the maximum force of static friction the block starts to slip and then the value for friction becomes kinetic friction (f_k) and the box is then under a net force so it accelerates to the right.

The force of friction depends upon both surfaces in contact and the normal force. A mathematical relationship can be created that relates the force of friction for any two surfaces to the normal force.

	In this first example, a block of wood is shown sliding across a wooden table. (notice the cause of this sliding is not shown) Notice that the force of kinetic friction (f_k) is equal to 40% of the normal force (F_N). Another way of writing this relationship would be
	If the weight of the block is doubled, the normal force doubles, and the force of friction becomes doubled. Once again we find that the force of kinetic friction (f_k) is equal to 40% of the normal force (F_N). Another way of writing this relationship would be Since this value is true for any weight of wood on wood, we say this value represents the coefficient of kinetic friction for wood on wood. The coefficient is the percentage of the normal force that can be delivered as friction. The formula for the coefficient of kinetic friction is That funny looking m or u is really the Greek letter "mu."
[QuickTime Version of all 3]	When looking at wood on asphalt, we find that the amount of friction on the block increased for for the same amount of weight. Notice that the force of kinetic friction (f_k) is equal to 60% of the normal force (F_N) or we could say

A different coefficient can be used to describe the relationship between the maximum force of static friction and the normal force. It's called the coefficient of static friction and its formula looks like

The maximum force of static friction is used because static friction has a whole range from zero Newtons up to the maximum force of static friction.

The coefficients for static and kinetic friction are listed in some reference tables. The coefficient of static friction is usually a little bit higher than coefficient of kinetic friction for the same two surfaces. When coefficients are listed they must be given for one surface on another surface (ie wood-on-asphalt). The higher the coefficient, the greater the force of friction. The table below lists the coefficients for a few common surfaces used in physics. They are arranged from "sticky" to slippery.

surface-on-surface
hook velcro-on-fuzzy velro	>6.0	>5.9
avg tire-on-dry pavement	0.9	0.8
grooved tire-on-wet pavement	0.8	0.7
glass-on-glass	0.9	0.4
metal-on-metal (dry)	0.6	0.4
smooth tire-on-wet pavement	0.5	0.4
metal-on-metal (lubricated)	0.1	0.05
steel-on-ice	0.1	0.05
steel-on-teflon	0.05	0.05
You should keep in mind that it isn't possible to give accurate values for the coefficient of frictions due to changing surface smoothness. For example, not all pieces of metal have the same surface smoothness. Some that are highly polished may be more slippery than others that are pitted or scratched. These values are just meant to give you the approximate values.