edcrater wrote:Imagine a planet made entirely of water [because it is incompressible], with a tube inserted from one side to the other, passing through the center of mass. In the tube is a ladder. On the outside of the tube is a line of handholds. An astronaut in a spacesuit starts climbing down inside the tube, and at the same time a diver starts pulling himself down the handholds, through the water. As the astronaut gets further down, the net gravity on him is less, due to there being some mass 'above', rather than all below. As he gets to the center, he is weightless. What about the diver? As he descends, the pressure on him increases due to the weight of water above. He experiences increasing pressure as he descends. As he gets deeper and deeper, there are 2 effects going on. The first is the increasing pressure with depth, and the second is that the gravity acting on a given volume of water is getting weaker, just as it is for the astronaut.
I suppose I should have read to the end of the thread before commenting. The above seems pretty apt. Granted, if there's air in the tube, it will also tend to increase in pressure. So, perhaps it would need to be a vacuum inside the tube (except the tube might get crushed at depth by the pressure of the water outside ;o])...
But, assuming it's a vacuum in the tube and it somehow miraculously doesn't get crushed at depth (perhaps it's made out of 'unobtainium' from the movie The Core ;o]), the diver would be subject to both gravity and pressure, whereas the guy in the tube would generally be subject only to gravity.
For the guy in the tube, gravity would decrease as he descends to the center point, since he'd have increasingly balanced amounts of mass on all sides which would cancel out in an equation.
For the diver, the gravity would decrease (as with the guy in the tube) while the pressure would increase (just like diving in Earth's oceans).
The more stuff you have stacked "above" you in a gravity well, the higher the inward "pressure" will be at your location.
~Michael