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Q & A: Pressure and holes in cans

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Most recent answer: 10/22/2007
Q:
We are doing work on the effects of air pressure on water. We took a can and made a hole in it. Then we measured the lenght of the water flow. Yet we are confused!! Is it the air pressure that is pushing the water out, or is it the pressure of the water above that is making the water come out of the hole at the rate it does??!!
- Meg and Christine (age 15)
NJ
A:
The water coming out of a hole in the side of a can near the bottom comes out quickly because the water pressure near the hole is bigger than the air pressure outside of the hole, and so there is a net force on a little bit of water that's passing through the hole.

The pressure of the water near the hole is the sum of the air pressure on the top surface of the water and the extra you get from the weight of the water. This pressure from the weight of the water is proportional to the depth from the surface of the water.

For most cans, if they are open, the air pressure on the top surface of the water is nearly the same as the air pressure outside the bottom near the hole. So the net pressure pushing the water out comes from the weight of the water in the can.

If the can is closed and you can push on the air inside with a piston, you can increase the pressure on the top surface of the water beyond what the air pressure is at the hole, and the water will shoot out faster -- you'll have made a squirt gun. Alternately, by pulling on the piston, you can reduce the air pressure on top so that the air pressure outside the can balances the pressure from the weight of the water plus the smaller pressure from the air on top. Water may dibble out of the hole. If you reduce the pressure on top further, you will suck air into the can through the hole and it will bubble up to the top of the can. If the hole is big, water may still splash out even in this case. If the hole is small, air can go in without water coming out.

Tom

(published on 10/22/2007)

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