# Water pressure and gravity

1. Dec 15, 2006

### Lelan Thara

I hope this is the best sub-forum for this question:

How direct is the relationship between water pressure and the weight of the water, as determined by the local gravity where the water is?

In other words - let's say we have a submersible with a crush depth of 1000 feet on Earth. If we moved that submersible to a watery planet with one fourth of Earth's gravity - would the crush depth become 4000 feet?

To phrase it another way - if I had a body of water large enough to swim in in a microgravity environment - would the water offer any resistance as I swam through it?

Do issues of surface tension, friction, viscosity and so so on play a significant role in determining water pressure? Or is it a simple direct correlation between water pressure and gravity - half the gravity means half the water pressure for equal volumes of water?

Thanks very much.

2. Dec 15, 2006

### Staff: Mentor

Direct, linear.
Yep. Since the pressure is simply the weight of the column of water above the submersible, if the water weighs one fourth as much, it can go four times as deep. Two caveats:

1. Gravitational force changes with distance from the center of a body.
2. Water is not completely incompressible.

On earth, anyway, these two considerations are essentially irrelevant, but they may not be on your hypothetical other planet.
Yes! (to both questions) Resistance to motion in water has nothing to do with the pressure of the water (same caveats as above, though). The resistance (and this goes for air too) is based on the density and viscosity of the fluid, and sometimes compressibility.

Surface tension, itself will only affect motion on the surface.

We may move this thread....

3. Dec 15, 2006

### kesh

i always wondered how pressure as a function of depth was affected by irregular containers. i keep thinking of sensible ways to calculate it that give absurd results in certain configurations

4. Dec 15, 2006

### Lelan Thara

Thank you, Russ. One further question about your statement below:

Since water is very slightly compressible - and that compression, in a natural deep water environment, would come from the weight of the water - would I expect somewhat less water resistance in a lower gravity environment?

5. Dec 15, 2006

### chroot

Staff Emeritus
Water is essentially incompressible, meaning that the difference in figures like "water resistance," by which I assume you mean drag, are only affected in a negligible way. You are correct, though, that the slight compressibility means that deep water is denser than shallow water, even with all other factors like temperature held constant. Denser water means, of course, more drag on a submersible vehicle. Again though, this effect is so tiny that it can be ignored, even on other planets.

- Warren