Game Development Reference
In-Depth Information
One of the consequences of friction is heat. When objects rub against each other, heat is
generated. Rockets reentering a planetary atmosphere will heat up due to friction between the
surface of the rocket and the colliding gas particles of the atmosphere. One reason your car
engine needs a cooling system is the heat generated due to friction between the various moving
parts of the engine.
Friction will be important to many game programming physical models. Obviously any
game that involves something sliding over a surface, hockey or shuffleboard, for example, will
have to include frictional effects. The friction between the golf ball and club head causes the
golf ball to spin and can cause the ball to hook or slice. Friction between the tires and the road
is what propels a car forward and keeps the car on the road when it drives around a curve.
In developing a mathematical relation to describe frictional force, we start with some
general observations about friction. For one thing, the amount of friction between two objects
depends on what the materials of the two objects are. The friction force between a metal box
sliding on a sheet of glass is less than that of the same box sliding on a piece of sandpaper.
Another general observation about friction is that it is proportional to the
normal force
between the contacting surfaces. The normal force exerted by an object is the component of
the force vector normal, that is, perpendicular, to the surface that is in contact with the object.
To compute the normal force between objects, we can make use of Newton's third law of
motion. If a box is resting flat on the ground, as shown in Figure 3-2, the normal force will be
equal and opposite to the gravitational force applied to the object.
mg
F
N
= mg
Figure 3-2.
Normal force on a box resting on the flat ground
The situation is a little different if the box is placed on a ramp inclined at an angle
q
as
shown in Figure 3-3. The normal force exerted on the box by the ramp is now perpendicular to
the surface of the ramp and has a value of
mg
cos
q
. Since friction is proportional to normal
force, we can conclude that friction will be less for a box on an inclined ramp than it will be if
the ramp were placed flat on the ground. In addition, the friction force will decrease as the
angle of the ramp increases.
In addition to the magnitude of the normal force, friction force is also a function of the
material properties of the two objects in contact with each other. The material property effects
are characterized by a quantity called the
coefficient of friction
. The value of the friction force,
F
F
, is equal to the coefficient of friction,
m
, multiplied by the normal force,
F
N
.
F
F
=
mF
N
(3.9)
