It need to be fast and it need to be fun
With all the objects set up and a way to detect the collision they now need to move according to the laws of physics and that's what rigid body dynamics do. This is based on the Newtons laws of motion but it really follow the laws of videogames physics. It need to be fast and it need to be fun. What fun is varies from milestone to milestone :(.
Constraints are use to restrict the movement of rigid bodies and create more advanced objects.
Use multiple constrained bodies to drive the pose of a character as it falls of a cliff.
Advanced Character Physics - 2003
An Introduction to Physically Based Modeling - 1997