If we were to calculate the force that one galaxy exerted onto another, would we consider the individual masses within the galaxies, or the masses of the galaxies as a whole?
Do the individual stars within one galaxy have control over the individual stars within another?
Or, do we consider the galaxies to be single bodies, each with it's own mass, and the distance between them as two uniform bodies?
If so, could we call galaxy clusters uniform bodies as well?
And if a galaxy cluster were to all of a sudden lose a star (hypothetically), would the net force exerted onto the other galaxy cluster be the same if you counted each star individually?
As another curiosity, I would like to pose this question: if a galaxy cluster were to lose one fourth of its mass, $2.0\times10^3$ light years away from the other galaxy cluster's centre of mass, how would the effect be different than if the mass was lost only $0.50\times10^3$ light years away?
What I mean is, the distance would be different, so the force would be different (**(if you can address this as well please)and information would have to travel faster than light for this to occur, yet with a mass loss this great, the other cluster would be sure to notice it almost immediately).