Basic Newtonian Mechanics - Physics for Game Programmers - page 34

The Coefficient of Friction
The value of frictional force between two objects depends on the coefficient of friction, m ,
between the contacting surfaces. If you've ever removed a cork from a bottle, you may have
noticed that initially you have to exert a lot of force to get the cork to move, but once the cork
begins to move it becomes easier to pull it out the rest of the way. Another way to demonstrate
this effect is to push a heavy box across the floor. It becomes easier to push the box once it
begins to move.
These observations highlight another characteristic about friction, namely that objects in
motion generally have a lower coefficient of friction than do objects at rest. When modeling the
effects of friction, two coefficients of friction must be taken into account. When the objects are
motionless, the coefficient of static friction, m s , governs the frictional force. When the objects
begin to slide, the friction between them usually decreases, and the frictional force is defined
by the coefficient of kinetic friction, m k . The values of static and kinetic coefficients of friction
for some material pairs are shown in Table 3-1. Unless otherwise noted, the coefficients are for
dry materials.

Table 3-1. Friction Coefficients for Some Common Surface Interactions

μ s μ k

Materials

Steel—steel 0.7-0.74 0.57-0.6

Steel—steel (lubricated) 0.12 0.07

Aluminum—steel 0.61 0.47

Copper—steel 0.53 0.36

Cast iron—cast iron 1.1 0.15

Teflon—Teflon 0.04 0.04

Glass—glass 0.94 0.4

Wood—wood 0.25-0.5 0.2-0.3

Rubber—concrete 1.0 0.8

Rubber—concrete (wet) 0.7 0.5

Ice—ice 0.1 0.03

Waxed ski—snow 0.1-0.14 0.05-0.1

www.roymech.co.uk
* Raymond Serway and John Jewitt, Physics for Scientists and Engineers, Sixth Edition (Brooks-Cole, 2003)
* www.physlink.com/Reference/FrictionCoefficients.cfm
* Encarta.msn.com

* Source: RoyMech,

Shuffleboard Game
To demonstrate the effects of friction, let's create a simple Shuffleboard Game simulation. The
game is implemented in a class named Shuffleboard . A typical screen shot for the game is