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Air pressure: water through a straw

  1. Jan 12, 2005 #1
    I vaguely remember a physics teacher once telling me that it is impossible to suck water through a straw that is longer than 32 feet because the air preassure is not strong enough to push it that high when you create a vacuum in your mouth. I know the mechanics behind sucking water through a straw, so that kinda makes sense to me.

    My question is, if it is true, would the straw have to be 32 feet high, or can it be on it's side across a table ( or something to that affect)?

    /edit for da_willem. my understanding of the mechanics was by no means worded correctly last time, please correct me if I am still wrong.
    Last edited: Jan 12, 2005
  2. jcsd
  3. Jan 12, 2005 #2
    Ofcourse it has to be 32 feet (actually 9,81m) high . Are you sure understand the mechanics behind sucking water through a straw?

  4. Jan 12, 2005 #3


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    The straw will work as long as the difference in height between the 2 ends is less then 32ft. When you reduce the pressure above the water in the straw, atmospheric pressure will push the fluid up into the straw. But the atmosphere can only support a column of water 32ft high. So the atmosphere can only push the water up 32ft.
  5. Jan 12, 2005 #4
    thanks for the verification Integral. Now, I'm going to find 33 feet of straw, and a 33 foot balcony to prove this to my friends who refuse to believe me
  6. Jan 12, 2005 #5


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    In other words, if you were a "perfect sucker" (there's one born every minute!) you could produce a perfect vacuum above the water: there would be no air pressure above it so air pressure on the surface of the water would push it up the straw. It can do that until the weight of the water in the straw (downward) is equal to the force of air pressure (upward). Yes, the straw has to be vertical.
  7. Jan 12, 2005 #6


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    Heck, just use a 5-foot (maybe 10 feet) straw (find some rubber or plastic tubing). Your lungs aren't anywhere near strong enough to suck a full vacuum.
  8. Jan 12, 2005 #7


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    Alas, your experiment is doomed. Even presuming you have olympic-level suction ability, the straw will collapse. Any material that is not strong enough to hold a vacuum will collapse.
  9. Jan 12, 2005 #8
    You can do this in water also. Get a pipe about 3 or 4 foot long and see how far you can go down under water and still breathe. You probabally wont make the 3 foot mark.
  10. Jan 13, 2005 #9
    Clart pipe
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