# Stuck between two large masses - does it get pulled apart?

by Peterfhannon
Tags: masses, pulled, stuck
 P: 3 If an object is between 2 large masses with high gravity, that haven't come together (gravity of other large masses is keeping them apart) does the object between them get pulled violently apart or does the gravity balance out?
 P: 615 Tidal forces are what cause objects to be broken up due to gravity. Basically, for an object that is relatively large, the force of gravity due to some other more massive object will be different at different points on the smaller object. Because gravity falls off with 1/r2, the tidal forces become more important the closer the objects are together. There is a distance from the large object called the Roche limit. The Roche limit is the distance from the large object where if the small object gets within it, it will start to break up due to the tidal forces. So basically it depends on how far away the objects are all from each other. An example of an object being bound between two more massive objects would be an object at the L1 Lagrangian point. An object at the L1 Lagrangian point would probably not be broken up due to tidal forces, because it would be so far from either of the larger masses. You can read more about those here: http://en.wikipedia.org/wiki/Lagrange_points
 P: 3 Thank you for your swift reply, I'm curious about whether gravity coming from 2 directions cancels out, or whether it pulls in both directions. I think you're saying that it cancels out. To clarify, what if something was in the hollow centre of a planet, would it be pulled apart or 'float' in the centre? As you can tell I'm not a physicist, just a curious layperson :).
P: 615

## Stuck between two large masses - does it get pulled apart?

Well, gravitational force is a vector quantity, which just means that it has a direction. If the gravitational force at a single point due to one object is equal in magnitude but opposite in direction to the gravitational force of another object, then yes, the net force on any object at that position will be equal to zero. This is the same idea as with the Lagrangian points.

As for an object in the center of a hypothetical hollow planet, you can find info on that in this article:http://en.wikipedia.org/wiki/Shell_theorem

basically what it says is that an object within a spherical, perfectly symmetrical shell of mass will feel no gravitational forces due to the shell of mass.

And also an object outside of the shell "sees" the shell as a single point of mass at its center.

It was shown by Isaac Newton using calculus.
 P: 3 Strange, thanks!

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