[Baby Astronaut]

If you skip to the 6:00 mark on Why Gravity Gets So Strong Near Dense Objects at Khan Academy, it shows how gravity pulls the interior of something outward from surrounding mass in all directions.

Wouldn't this form a hollow region in the center of stars and planets? Isn't the gravity in that part the weakest compared to the rest of its body?

[Cap'n Refsmmat]

When you're at the center of a spherical object, all the mass around you pulls outwards, but in opposing directions. The net force is essentially zero. Hence if you were to build a small chamber at the exact gravitational center of the Earth, you'd float inside it.

[Xittenn]

Baby Astronaut, on 8 February 2012 - 11:48 PM, said:

Wouldn't this form a hollow region in the center of stars and planets? Isn't the gravity in that part the weakest compared to the rest of its body?

As Cap' has mentioned, but in addition there are external pressures crushing in on that central area. So even though the net gravity is weakest at that point, points further out radially are being pulled inward toward the center. All of these other points of mass being pulled inward ensure the fact that the center will not be hollow.

This post has been edited by Xittenn: 9 February 2012 - 02:30 AM

[D H]

Cap, on 9 February 2012 - 01:48 AM, said:

The net force is essentially zero.

The net gravitational force is zero. The pressure, however, is immense. A hollow region as mentioned in the original post couldn't exist because of the immense pressure.

[Baby Astronaut]

Isn't the pressure solely from gravity itself?

Now it makes sense though. At a certain distance from the Earth's crust, you have 90% of the planet's matter pulling on that specific zone towards the center (not all directly towards it but a lot in that general direction) and past it. Lower still, the pull becomes 80%, but now 20% (from above) is pulling it outward, but that 20% itself is also being pulled inward by the 80% adding further inward pressure onto the zone. Then 70% and 30% with 70% pulling on the 30, then 60% and 40% with 60% pulling on the 40, then 50% (dead center) with the opposite 50% pulling on this 50% and that 50% being pulled onto the same area by this 50%.

Or something like that.

And yes I realize that both sides are pulling each other inward no matter what the percentage ratio is. But is that analysis very far off conceptually? (I know the math's horribly inaccurate and basically nonexistent)

[D H]

Baby Astronaut, on 10 February 2012 - 12:13 AM, said:

Isn't the pressure solely from gravity itself?

Yes, it is.

Pressure is essentially the weight of all of the mass above the point in question. So even though gravitational force goes to zero at the center of some object, the pressure goes to a maximum at the center.