OBJECT PROPERTIES

Physical objects have properties that affect their movement and interactions with other objects. We’ve already talked about mass, position, velocity, force, the inertia tensor, angular velocity, and torque. These properties describe object motion under force in free space. When objects bump into each other or into infinitely heavy objects, their reactions are dependent on three more properties: restitution, static friction, and dynamic friction.

Restitution is the amount of bounce that an object has when it hits something and is usually expressed in a positive floating-point number. A good way to think of this is how high a ball will bounce when you drop it. If the restitution is 0.0f, you’ve got a piece of playdough, and when it hits it will simply stick to the ground. If you’ve got something like 0.99f, you’ve got a nice superball that will bounce around for a long time. It’s a bad idea to assign restitutions of greater than 1.0f, since the object will simply continue to gain energy forever.