Game Development Reference
F IGURE 15.4
A microscopic view of static and dynamic friction.
When the object is moving, the friction acts as dynamic friction, acting against the
direction of motion.
The friction model we'll develop in this chapter will follow this model and com-
bine both types of friction into one value. In what follows I will point out where we
are using static friction and where it is dynamic, so you can replace the single value
with two values if you need to.
There is one further type of friction that is important in dynamic simulation. Rolling
friction occurs when one object is rolling along another. It is most commonly used for
high-quality tire models for racing simulations (in the sense of simulations performed
by motor-racing teams rather than in racing games).
I have not come across physics engines for games with a comprehensive tire model
that includes rolling friction. I have worked with one non-game physics engine that
included it, however. Because we are focusing on game applications, I will ignore
rolling friction for the rest of the topic.
I SOTROPIC AND A NISOTROPIC F RICTION
There is one additional distinction between types of friction that we need to recog-
nize: friction can be either isotropic or anisotropic. Isotropic friction has the same