# Thread: Question : Increasing mass and the gravitational field

1. Imagine this simpple case :

A spherical mass with rather low density under a gravitational condition,
resulting in a certain g factor at the surface and a dropping g-factor away from the sphere
according to Newton's inverse square law.
A gravitational field is formed of a certain strenght.

Now imagine that i could gradually insert into the sphere an amount of extra mass,
increasing thus gradually the density and the mass of the objekt, and the strenght of the gravitational field.

For instance mass x2 in the same volume > g at the surface x2, dropping in value further away.

> What is fysically happening then during this event, to cause the gravitational field to get stronger ?

We know the formulas for what is happening, we observe the resulting behaviour of the objekts,
but how could we explain this transition as a process ?  2.

3. What is fysically happening then during this event, to cause the gravitational field to get stronger ?
You are increasing the mass that is present. This in turn increases the amount of space-time curvature. (Or, perhaps more accurately, the mass is the space-time curvature.)  4. Originally Posted by Noa Drake according to Newton's inverse square law.
That's true only if the mass is small, has no angular momentum, and no net electric charge.

but how could we explain this transition as a process ?
Not sure what you are getting at. The physical process is that you are adding energy-momentum to the system, so the gravity changes as a result.  5. Bear in mind that the extra mass has to come from somewhere, and that this mass always had a gravitational field associated with it. Then this gravitational field would simply combine with the original gravitational field to increase the total gravitational field.  6. Originally Posted by KJW Bear in mind that the extra mass has to come from somewhere, and that this mass always had a gravitational field associated with it. Then this gravitational field would simply combine with the original gravitational field to increase the total gravitational field.

Indeed a good remark.

The intriguing thing there is, observing the force of 2 near masses, that 'combining' here is not 'adding' (summation of m1 and m2)
but 'multiplication' (m1 x m2) leading to F.

F = G x m1 x m2 / R²

Say

m1 = 2

m2 = 6

> F ~ 12 (not an added 8)

Obvously because the formula says so, and empyrically correct, but is it also obvious from observing the 2 gravitational fields ?

> I would answer like this :

m1 is present in a field that makes up the base state for the gravitational field of m1,
and that field is produced by a mass m2, being 6, thus laying out the base state for m1 with factor 6
Hence : 2 x 6 ~ 12 for F

And similarly the other way :

m2 is present in a field that makes up the base state for the gravitational field of m2,
and that field is produced by a mass m1,being 2, thus laying out the base state for m2 with factor 2
Hence : 6 x 2 ~ 12 for F

Is that a correct way of looking at it ?  7. Originally Posted by Noa Drake Indeed a good remark.

The intriguing thing there is, observing the force of 2 near masses, that 'combining' here is not 'adding' (summation of m1 and m2)
but 'multiplication' (m1 x m2) leading to F.

F = G x m1 x m2 / R²

Say

m1 = 2

m2 = 6

> F ~ 12 (not an added 8)

The formula you give is for finding the force acting between two masses at a given distance from each other, not for the force you would get from combining two masses. IOW, it is for the force at which they pull at each other.

You multiply the masses because the gravity of each mass acts on the other. Mass 1 pulls on Mass 2 and vice versa.

To use this formula to determine how the gravity field would change for an object as its mass changes, you set one of these masses to a set value, say 1, and then add to the other.

Thus if you start out with a mass of 2, you get 2x1 = 2.

If you increase the mass by 6, you get (2+6)x1 = 8  8. Originally Posted by Janus   