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Q & A: Pressure - Gravity Matters?

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Most recent answer: 10/22/2007
Do you know any experiments to show how pressure and gravity are related?
- Rhea (age 14)
Van Nuys
Rhea -

Sure. You probably already know that air pressure goes down at high altitudes. That's because it's gravity that's holding the air near the Earth. You can think of it this way - the pressure at some height has to be enough to support the weight of all the stuff above it. The higher you go, the less stuff is above you, so the lower the pressure.It's a nuisance to travel to some other altitude to check that. Since water is a lot denser than air, in water the pressure changes a lot even for small height differences. Here's how you can use water to see the way gravity affects pressure.

Get an empty 2-liter soda bottle and poke several small holes in it going up the side of the bottle. Plug all of the holes up (you may be able to do this with the needles that you used to make the holes). Fill the bottle all the way up and pull the plugs out. The water will shoot out of the holes because of the pressure of the water pushing out. The higher the pressure of the water, the farther the water will shoot. What you will probably see is that the water coming out of the bottom holes will shoot farther than the water from the top holes. The pressure of the water is higher as you go deeper into the soda bottle. Its the same reason you feel the pressure on your body (in particular your ears) rise as you dive to the bottom of a pool.

This is also the reason that human divers can't go below a certain depth in the ocean. For humans, pressures of more than 20-80 atm can cause problems, which is equivalent to about 650-2600 meters underwater. Although humans generally don't try to dive this deep, some seals have been known to dive more than 650 meters below sea-level, so that they have to have special adaptations to let them survive at high pressures. For more info on exactly how to calculate all these numbers, check out the answer to the question .


(published on 10/22/2007)

Follow-Up #1: Deep water diving dangers

what are some problems that divers may encounter when they dive too deep?
- elvie (age 16)
Other than sharks and drowning, the most significant danger of deep water diving is
"the bends".  The cause is that under high pressure (1 atmosphere for every 33 feet) the blood starts to absorb nitrogen gas.  When the diver surfaces quickly the nitrogen gas comes out of the blood in the form of bubbles.   These bubbles raise havoc with the blood circulation and can cause intense pain and even death.   

 Example: when you open up a bottle of carbonated water or beer, you see a lot of bubbles and foam. Same principle only it's carbon dioxide instead of nitrogen.

There is a nice article in Wikipedia
that has a lot of details.


I'll bet that numerically sharks aren't the top danger.  Mike W.

(published on 10/22/2007)

Follow-up on this answer.