In my notes, it says that the total change in momentum of an inertial reference frame is zero. Please see the picture below
This has confused be because
I nnderstand that the inertial reference frame itself, by definition, cannot be accelerating. So for example if someone is looking out of their accelerating car, what they see is not an inertial reference frame.
however I do not understand that the total change in momentum will be zero. If someone is looking out of their car now moving at constant velocity (or stationary or whatever! It is now an inertial reference frame), could they not see an object that they are viewing in their inertial reference frame be accelerating because there is a net force acting of it? For example, they are watching an accelerating train which is accelerating in their own inertial reference frame?