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A system of N particles experiences no external forces. Which statement about the magnitude of the system's total momentum is true?

1. It must be zero
2. It could be non-zero, but it must be constant
3. It could be non-zero, and it might not be constant
4. The answer depends on the nature of the internal forces in the system

Correct answer: It could be non-zero, but it must be constant

Solution

With zero net external force, total momentum is conserved, so it stays constant; but it need not be zero (the centre of mass can move at constant velocity). Internal forces do not affect total momentum.

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