Game Development Reference
In-Depth Information
get these priorities confused and end up creating an impressive technical
demo that isn't any fun.
12.2.2
Frictional Forces
If we take an object such as a bowl of petunias and slide it along a surface,
we know that it will eventually come to a stop. We also know that if we
place this bowl on a surface that isn't quite level, it won't necessarily slide
downhill unless the angle of inclination exceeds a certain threshold. These
two phenomena are slightly different aspects of the force of friction. We are
accustomed to thinking of friction as an onerous enemy of productivity, the
evil cause of wear on machines and more frequent trips to the gas station.
But keep in mind that without friction, we wouldn't be able to walk across
a room or pick up a child (or a bowl of petunias). Without friction, our
cars might have better fuel e ciency, but the transmission wouldn't work
and the tires would spin in place instead of propelling the car forward.
Here we consider the two modes of the standard dry friction model,
which is sometimes called Coulomb friction. Although several thinkers
contributed to our understanding of friction, Charles-Augustin de Coulomb
(1736-1806) is the guy who got his name to stick. When an object is at
rest on top of another object, a certain amount of force is required to get
it unstuck and set it in motion. If any less force is applied to the object,
the force of friction will push back with a counteracting force up to some
maximum amount. This type of friction is known as static friction, and it
prevents bowls of petunias sitting on slightly inclined tables from sliding off.
Once static friction is overcome and the object is moving, friction continues
to push against the relative motion of the two surfaces, but the magnitude
of this force, known as kinetic friction, is less than that of static friction.
Kinetic friction is what causes a bowl of petunias to eventually come to a
stop after we set it in motion.
Friction is the result of complicated interactions at the microscopic level,
and so it is somewhat surprising that its macroscopic behavior can be de-
scribed by relatively simple equations. Let's consider static friction first.
Like any force, static friction is a vector. The direction of static friction is
always in the direction that opposes any forces that would otherwise cause
objects to move relative to each other. This might seems a bit like cheating
(“How does the friction always know the correct direction to push?”), but
remember that the force is actually the aggregate result of many electrical
forces acting at the microscopic level. The forces are the result of molecular
bonds that have formed between the objects as they came in contact, and
these bonds need a force to pull them apart.
A good approximation for the maximum magnitude of static friction is
computed with Equation (12.5).
 
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