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## Lecture 3: Dynamics

Lenny: “What makes things move, George?

George: “Forces do, Lenny.”

Lenny: “What makes things stop moving, George?”

George: “Forces do, Lenny.”

### Aristotle’s Law of Motion

Aristotle lived in a world dominated by friction. To make anything move—a heavy cart with wooden wheels, for example—you had to push it, you had to apply a force to it. The harder you pushed it, the faster it moved; but if you stopped pushing, the cart very quickly came to rest. Aristotle came to some wrong conclusions because he didn’t understand that friction is a force. But still, it’s worth exploring his ideas in modern language. If he had known calculus, Aristotle might have proposed the following law of motion:

The velocity of any object is proportional ...

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