Air pressure: water through a straw

  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. Ofcourse it has to be 32 feet (actually 9,81m) high . Are you sure understand the mechanics behind sucking water through a straw?

  4. Integral

    Integral 7,288
    Staff Emeritus
    Science Advisor
    Gold Member

    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. 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. HallsofIvy

    HallsofIvy 41,266
    Staff Emeritus
    Science Advisor

    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. russ_watters

    Staff: Mentor

    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. DaveC426913

    DaveC426913 16,541
    Gold Member

    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. 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. Clart pipe
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