In previous sections, you have mostly been using
The Particle Model: assuming objects act like point particles. A principal feature of this assumption is that the objects do not rotate. You will now relax this assumption to allow an object to rotate, but only if the entire object rotates with the same angular velocity. Such an object is typically known as a rigid body.
Assumption 5.9.1. The Rigid-body Model.
When you assume that a system can be treated as a rigid body, you treat it as undergoing both translational motion and rotational motion. Under this assumption, you treat all parts of the system as moving with the same angular velocity, while the center of mass of the object is treated as moving with the translational velocity.
Exercises Activities
1.
Give an example of a context where an object does not behave like a rigid body. What aspect of the assumption is not valid for your example?
2.
Suppose you have two objects: a wheel that is both moving and rotating, and a block that is moving without rotating. Is it possible to choose a system consisting of both the wheel and the block? Why or why not?
Answer.
Yes, it is possible! The two objects will use different models---treat the block as a point particle and the wheel as a rigid body---but any objects can be included in the same system. Interesting, neither the Point Particle model nor the Rigid-body Model apply to the system as a whole, but knowing how to treat each aspect of the system is enough for you to describe how it will behave physically.
