Question

When is the total momentum of a system conserved?

Answer 1

When the net external force is zero

Step-by-step explanation:

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

`F=(\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.