Conservation of Momentum: Feynman's Physics Explained
Newton's third law says that for every action there is an equal and opposite reaction: push on me and I push back just as hard. A wonderful consequence is that the total momentum — mass times velocity — of an isolated system never changes, because all the internal pushes cancel. This lets us analyze a car crash or a rocket launch without knowing the messy details of the forces: the total momentum before must equal the total momentum after.
The big idea
In any closed system, total momentum is preserved.
Think about it
A rocket has nothing to push against in empty space. How does throwing exhaust backward make it move forward?
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