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so here goes...

Newtons law of gravity, is regarding the force of a gravitational field on 2 objects. But the equation only asks for the mass of the two objects. so.....

1. does size not matter? ie: an earth sized object with earths mass and a grapefruit sized object with earths mass would give the same results? if yes why are black holes such a big problem?

2. there is no way to work out a gravitational force of just one object? ie: the equation asks for two masses divided by the distance. so you can't work out an objects gravitational force without having another mass act upon it? again if yes can anybody really say an object has a gravitational field if it takes two objects?

the second part of my query and what really is confusing me at the moment is this:

a star with the mass of 'X' has an effective gravitational field that extends 'N' light years (for example).

the star is in a cluster of 10 million stars each with a mass of 'X' and an effective gravitational field that extends 'N' light years.

is the clusters gravitational field based on 10 million individual gravitational fields or is it based on a cluster with the mass of 'X' times 10 million. And if the latter would the clusters effective gravitational field extend proportionally (ie: 10 million times 'N' light years).

given that a gravitational field is infinite but it's effective range is a range that can capture a slow moving object of 'P' mass. ie: don't turn that point into an argument. you'll either get what i mean or you won't.

anyway, any enlightenment here would be gratefully appreciated and I'll warn you now will probably lead to follow up questions.