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Name: Alex M.
Status: N/A
Age: 60s
Location: N/A
Country: N/A
Date: 1/13/2004

It is generally accepted that there is a null gravity point at the centre of the earth. It follows logically that this is probably true of any object in free space ranging from cosmic dust to black holes. Does this suggest that the sun's nuclear fusion could not occur at its centre (lack of pressure)? and that a black hole must have an internal event horizon? surely gas would also get trapped at the centre.

I am not certain I understand what you mean by a "null gravity point". Certainly the forces of gravity are balanced at the "center" so that any object at that point is pulled equally in all directions, resulting in a weightless condition. However, that is not the same as saying that the things stacked on top of you are not pushing down on you from all directions.

A comparable situation would be a nut in a nutcracker. The nut experiences no net force (it does not accelerate away from or toward the nutcracker) but it experiences enough pressure to crack the nut.

In the same way, if you were to suddenly appear in the center of the black hole you would not experience a force that would accelerate you away from the center but you would certainly be crushed by the extreme pressure there.

Greg Bradburn

No, the lack of a *net* gravitational force at the center of the sun does not say anything about the pressure there, but we know it is huge because of the weight of all the stuff on top of it.

We do not know much about the inside of a black hole.

Tim Mooney

Alex M.,

Pressure and force are not the same thing. At the center, gravitational force is essentially zero. The net force is also zero. Still, there is great force. The force is due to adjacent material pushing in on the material at the center. It is being pushed from all directions. The net effect does not have a specific direction. Still, the forces from all around do compress the center.

A good example is an empty can being pressed equally from both ends. The can is pushed neither to the left or the right. Still, the can eventually collapses.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College

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