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For a system of moving particles, which statement is correct regarding the sum of the magnitudes of the individual particles' momenta?

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. It could be zero, even if the magnitude of the total momentum is not zero

Correct answer: It could be non-zero, and it might not be constant

Solution

The scalar sum of momentum magnitudes is generally non-zero and is not a conserved quantity, since particles can change speeds and directions even while total vector momentum is conserved.

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