Weight of Water

Imagine an inverted test tube with a column of water in it. The volume above the water is empty (no air). The piston underneath is free to move up and down with no friction. If the water is not moving, what can you say about its weight?

a way to think about air pressure…

vacuum

water

piston

Weight of the water pushes on the piston from above.

Air pressure pushes from below.

The force from the air pressure must exactly cancel the

weight of the water!

Q: How tall is the column of water that exactly balances atmospheric air pressure?A: ≈ 10 mair

vacuum

10 m of water

piston

air

Pressure is 1 atm.

Pressure is 0 atm.

Pressure is 0.5 atm.

Remember: pressure doesn’t depend on the diameter of the tube. It only depends on the height of the column of water.

A siphon works because of a difference in pressure.

Q: What if the siphon were 100 m high?A: Wouldn’t work. Can’t go above 10 m.

Equal Siphons

Remember: pressure doesn’t depend on the diameter. It only depends on the height.

These two siphons work the same way. They don’t depend on the weight of the water on each side. They just depend on the pressure, which pushes.

Same difference in height leads to the same difference in pressure.

… unlike a string of beads

More weight on this side

Less weight on this side

More weight on one side pulls the string of beads to that side.

The wall can be as high as you like, as long as the string doesn’t break.