If you throw a baseball up on the moon, and it comes down, how is momentum conserved?
Momentum of the "baseball-Moon" system will be conserved, but the momentum of the baseball alone would not.
How? I don't understand.
How what? What specifically don't you understand?
What are the conditions under which momentum is conserved?
Does the moon gain momentum in the direction the ball was going?
The moon and baseball exert equal and opposite forces on each other. If one gains momentum in one direction, the other must lose momentum in that same direction (or gain it in the opposite direction).
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