Game Physics Engine Development, 2nd Edition

128 Chapter 7 Hard Constraints

Velocity and the Contact Normal

When we have two objects in resting contact, we are interested in their relative velocity

rather than the absolute velocity of either. The two objects might be in resting contact

with one another in one diretcion, but moving across each other in another direction.

A box might be resting on the ground, even though it is skidding across the surface at

the same time. We want the vibrating contacts code to cope with pairs of objects that

are sliding across one another. This means we can’t use the absolute velocity of either

object.

To cope with this situation, the velocity and acceleration