Game Development Reference
In-Depth Information
In Figure 3-1 , the block is resting on the horizontal surface with a small force, F a , applied
to the block on a line of action through the block's center of mass. As this applied force
increases, a frictional force will develop between the block and the horizontal surface,
tending to resist the motion of the block. The maximum value of this frictional force is:
F fmax = µ s N
where µ s is the experimentally determined coefficient of static 1 friction and N is the
normal (perpendicular) force between the block and the surface, which equals the
weight of the block in this case. As the applied force increases but is still less than F fmax ,
the block will remain static and F f will be equal in magnitude to the applied force. The
block is in static equilibrium. When the applied force becomes greater than F fmax , the
frictional force can no longer impede the block's motion and the block will accelerate
under the influence of the applied force. Immediately after the block starts its motion,
the frictional force will decrease from F fmax to F fk , where F fk is:
F fk = µ k N
Here k means kinetic since the block is in motion, and µ k , the coefficient of kinetic
friction, 2 is less than µ s . Like the static coefficient of friction, the kinetic coefficient of
friction is determined experimentally. Table 3-1 shows typical coefficients of friction for
several surfaces in contact.
Table 3-1. Coefficients of friction of common surfaces
Surface condition
Μ s
Μ u
% difference
Dry glass on glass
0.94
0.4
54%
Dry iron on iron
1.1
0.15
86%
Dry rubber on pavement
0.55
0.4
27%
Dry steel on steel
0.78
0.42
46%
Dry Teflon on Teflon
0.04
0.04
Dry wood on wood
0.38
0.2
47%
Ice on ice
0.1
0.03
70%
Oiled steel on steel
0.10
0.08
20%
The data in Table 3-1 is provided here to show you the magnitude of some typical friction
coefficients and the relative difference between the static and kinetic coefficients for
certain surface conditions. Other data is available for these and other surface conditions
1. Static here implies that there is no motion; the block is sitting still with all forces balancing.
2. The term dynamic is sometimes used here instead of kinetic .
 
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