Answer:
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
[tex]F=\frac{\Delta p}{\Delta t}[/tex]
where
F is the net external force acting on a system
[tex]\Delta p[/tex] is the change in momentum of the system
[tex]\Delta t[/tex] is the time interval
When the total momentum of a system is conserved (so, it does not change), its variation is zero:
[tex]\Delta p = 0[/tex]
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.
