# Is gravity strongest in center?

1. Apr 1, 2013

### Boy@n

Is gravity, in sense of pressure, strongest in center of objects like planets, stars, BH, galaxies?

If all atoms attract each other then center cannot be it, right?

If you had a hole in center of Sun (hypothetically) and put there a little ball, would that ball get crushed, torn apart or just stay there unaffected because gravity cancels itself out in centre?

So, wouldn't an object feel greatest pressure somewhere half way to the centre?

If this holds for a star, what about a galaxy, if same logic applies why would galaxies have most massive black hole in their center rather say half way there?

2. Apr 1, 2013

### ghostanime2001

Good question that question needs an application of quantum mechanics and cannot be explained by merely from classical mechanics.

3. Apr 1, 2013

### mathman

You seem to be mixing two different effects at the center (ignoring quantum altogether), pressure due to stuff on top and gravity. There is no gravity at the center, but the pressure is enormous.

As for the galaxy question, most (if not all) galaxies have a massive black hole at the center.

4. Apr 1, 2013

### Bandersnatch

(@ghostanime2001: surely you jest? April Fools', eh?)

@Boy@n:

What does it mean "gravity in the sense of pressure"?

At the centre of any massive body, the force of gravity is the lowest(i.e., 0), while the pressure is the highest.

5. Apr 1, 2013

### Boy@n

Why is pressure the highest in centre if gravity is zero there?

I'd imagine all atoms (which are making up the object) pulling from all directions, so, atoms on the surface pull up those deeper down, thus I'd imagine pressure to not be highest in center...

So gravity of BH is zero too since it is a singularity?

Last edited: Apr 1, 2013
6. Apr 1, 2013

### Boy@n

Doesn't pressure happen due to gravity? No gravity no pressure right? I am a bit aware of quantum, or say gravity causing curvature of spacetime but I don't think considering that would help me answer a simple (maybe even silly) question in your eyes... But since we mentioned it - is spacetime curved in layers for a massive object? In centre of it the most?

7. Apr 1, 2013

### Bandersnatch

The Shell Theorem explains why gravity is falling the deeper you go:
http://en.wikipedia.org/wiki/Shell_theorem#Inside_a_shell

In short, once inside, the gravitational forces from matter constituing a shell of matter above cancels each other in every point inside the shell.
So the only thing that actually attracts is whatever is underneath one's feet.

As for the pressure, since each and every consecutive shell is attracted to the centre of mass with at least some force, and nothing can be cancelled here, these influences can only add with the highest value being at the centre.

It's non-zero over singularity.
And since the singularity is point-like, you can't ever be not over it.

8. Apr 1, 2013

### Boy@n

Thanks for all explanations!

9. Apr 1, 2013

### P.Bo

There's no net force due to gravity at the center (assuming something with spherical symmetry), because the gravity is pulling on you in every direction. If you were at the center, the "stuff" on top of you would not be at the center, would have net gravity affecting it would still pull it toward the center, albeit it very lightly due to lower gravity, but the weight of all the stuff on top of that all the way to the surface where the gravity is the greatest would still be pushing down on you... hence pressure.

10. Apr 1, 2013

### Boy@n

Now I don't think I get it anymore... If there would be no gravity in existence would there still be pressure? In my understanding not...

If gravity is strongest on the surface it should pull everything from centre up to the surface thus pressure wouldn't be highest in centre... I'll have to read other sources, seems I cannot see the connection between gravity and pressure.

Last edited: Apr 2, 2013
11. Apr 2, 2013

### A.T.

If gravity and pressure would be the same thing, why would we have two diferent names for it? If there was gravity at the center, which direction would it pull?

12. Apr 2, 2013

### P.Bo

Pressure is simply a force that's applied over an area, in the sense you/we are talking about it is caused by gravity creating a force, weight, and that force applied over an area translates to pressure. If there was no gravity anywhere, there would be no force.

However at the exact center of something is the only place there would be no net force due to gravity, there would still be a force due to gravity at everything not at that center, and all of it is still attracted to the center. So you would still have the weight (due to gravity) of all that stuff on top of you in your example of the center of the planet/sun/etc, as a result the pressure is greatest.

You're misunderstanding when I say gravity is the strongest on the surface, the force of gravity is strongest on the surface due to all the stuff causing that gravity being beneath it. So whatever is on the surface is going to feel the strongest pull towards the center because there is the most amount of stuff "beneath" it (and it can be as close as possible). It doesn't mean the force of gravity pulls up towards the surface.

13. Apr 2, 2013

### Boy@n

They are not the same thing yet pressure happens due to gravity, no?

I can see there is no gravity in centre (being canceled out), I just don't see why pressure is highest there?

I imagine it's highest half way to center...

It's obvious that my imagination and my logic fails me, since facts show that in centre pressure is highest...

I am not saying anyone is wrong here, I am just saying I don't see how the two work together, and would like to improve/correct my understanding.

Will re-read all replies here and see if I experience 'engravitment' (silly joke, couldn't say enlightenment ;-)

14. Apr 2, 2013

### Boy@n

P.Bo, your reply clears up a few things for me, thank you. My reply above was to A.T. before seeing yours...