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Real-world systems often involve rotating objects or objects which must be prevented from rotating. In some cases, the rotation is constrained to occur about an obvious axis (e.g. a door will rotate about the line defined by its hinges, a fixed pulley will rotate about its axle, etc.). In some cases, however, there is no obvious fixed axis of rotation. For example, a rolling ball or wheel translates and rotates, and a collision involving a rotating object will often result in a change in the "physical" axis of rotation. In these cases, it is important to understand the rules for choosing an axis that will allow (and ideally, simplify) analysis of the situation.
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Common Choices of Axis
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