Question

When is the total momentum of a system conserved?

Answer 1

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

$F=\frac{\Delta p}{\Delta t}$

where

F is the net external force acting on a system

$\Delta p$ is the change in momentum of the system

$\Delta t$ is the time interval

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

$\Delta p = 0$

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.