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A ball is thrown vertically upward from the surface of the Earth. Considering the Earth-ball as an isolated system (ignoring air), which statement best describes the motion of both during the ball's upward journey?

1. The Earth stays stationary while the ball moves upward.
2. The ball stays stationary while the Earth moves downward.
3. The ball and the Earth move toward each other.
4. The ball and the Earth move away from each other.

Correct answer: The ball and the Earth move away from each other.

Solution

In an isolated system, total momentum remains conserved. When the ball moves upward, the Earth recoils slightly downward, so the separation between them increases during the ascent.

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