Mechanics 2: Linear and Rotational Dynamics - 3D Math Primer for Graphics and Game Development - page 568

Static Friction

f s = s n. (12.5)

The dimensionless constant s is known as the coe cient of static friction,
and n is the magnitude of the normal force. Let's talk about each of these
in more detail.
From our perspective, s is certainly the easier of the two to deal with:
just look it up in a table! Table 12.1 shows just such a table. Note that we
are jumping ahead a bit and showing coe cients for both static and kinetic
friction. Ignore the kinetic friction column for now.
Of course, somebody actually has to fill out these tables! The methods
for obtaining these data are interesting and rather elegant but they are
not our primary concern here. What is very important for us is that the
coe cients of static and kinetic friction depend on the properties of both
interacting surfaces. In other words, Table 12.1 is indexed not by a single
surface type, but by a pair of interacting surfaces. So, for example, although
using this table we can find the coe cient of static friction for rubber
against asphalt, we cannot use this information to say anything about, for
example, rubber against ice, or wood against asphalt. The coe cient of
static friction for each pair of surfaces has to be measured experimentally
because of the complexity of the microscopic interactions.
Also, note that Equation (12.5) tells us the maximum strength of the
static friction force. The actual force exerted at any instant will meet the

Material 1 Material 2 µ s (Static) µ k (Kinetic)

Aluminum Steel 0.61 0.47

Copper Steel 0.53 0.36

Leather Metal 0.4 0.2

Rubber Asphalt (dry) 0.9 0.5-0.8

Rubber Asphalt (wet) 0.25-0.75

Rubber Concrete (dry) 1.0 0.6-0.85

Rubber Concrete (wet) 0.30 0.45-0.75

Steel Steel 0.80

Steel Teflon 0.04

Teflon Teflon 0.04

Wood Concrete 0.62

Wood Clean metal 0.2-0.6

Wood Ice 0.05

Wood Wood 0.25-0.5

Wood (waxed) Dry snow 0.04

Table 12.1. Static and kinetic coefficients of friction