When is the total momentum of a system conserved?

When the net external force is zero

Explanation:

We can answer to this question by referring to Newton's Second Law, which can be written in the following form

where

F is the net external force acting on a system

is the change in momentum of the system

is the time interval

When the total momentum of a system is conserved (so, it does not change), its variation is zero:

In order to satisfy this condition, we see from the formula that we must also have

F = 0

so the net external force acting on the system must be zero.

the answer is c i did the same question so i know that's right!

the correct choice is a .

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