Up to this point, we have modeled all objects we have looked at as point particles.  Regardless of the actual extent of the particle, we have treated, boxes, balls, cars, people, even planets, as if they occupied a single point in space.  We have been able to do this because how the mass of an object is distributed hasn’t mattered in the scenarios we have previously examined.  But there are obviously situations where the way the mass of an object is distributed does matter, influencing both how forces act on the object and the object’s resulting motion.  To look at these scenarios, we must start by introducing a new model which we can use to describe objects with more complicated motion.

Though there are many ways in which we could choose to make an object more complicated, let’s begin by treating extended objects as rigid bodies, which do not deform as they move.  As an example, a pulley with a string running across it acts like a rigid body because it doesn’t change its shape as it rotates about its axis.  What our new model will allow us to do is to describe not just the translational motion of an object (which is what we have done up to now), but also how the object rotates about some axis.