The general motion of a rigid body can be completely described by
the linear motion of its **center of mass** and its rotation about
the center of mass.

Newton's Second Law therefore has two parts:

**Linear Motion**

where is the net force on the object,

**Rotational Motion**

where is the net torque on the object,

**Note**

- When applying Newton's laws to solve for the motion of a system of objects, one must apply the rotational version of the Second law to each extended object in the system that is free to rotate.
- In the absence of net external torque the total angular momentum of any system of objects (about any axis) is conserved.

2001-01-05