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Giant Straw Question

  1. Sep 14, 2009 #1
    This is a dumb question that my coworkers and I are debating, and we can't come up with an answer.

    Say one were to build a giant straw with one capped end that extends from sea level into space, out past Earth's gravitational field. What happens when the end is uncapped? Does the atmosphere of Earth get sucked out into space? Or does Earth's gravity overpower the suction of the vacuum?

    Just curious.
  2. jcsd
  3. Sep 14, 2009 #2
    Imagine you gradually build a hollow cylindrical tower from sea level to space. When you reach the upper edges of the atmosphere, does the air suddenly rush out of the tower? No, you've essentially just built a wall around a stably existing column of air, which isn't going to make it suddenly unstable. But this is the same thing as the straw.
  4. Sep 14, 2009 #3
    Nothing would happen. A vacuum doesn't exert any pulling force. What most people call suction is the absense of atmospheric pressure. A beverage rises up a drinking straw because at the bottom of the straw atmospheric pressure pushes upward on the liquid, and at the top of the straw the atmospheric pressure has been removed, leaving no downward force to oppose the upward force.
    Last edited: Sep 14, 2009
  5. Sep 14, 2009 #4
    Nothing will happen. Whether you build it in our atmosphere and extend it into outer space or build it in space and extend it into our atmosphere...the Earth's atmosphere and Space's vacuum will not breach each other. The area inside the straw will equalize.
  6. Sep 14, 2009 #5


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    Staff: Mentor

    One useful way to think of air pressure is that it is the weight of a column of air of a given cross sectional area, extending from sea level to space. (ie, the weight of a 1 sq in. column of air is 14.7 lb) Whether that column of air is enclosed in a pipe or not is irrelevant.
  7. Sep 14, 2009 #6
    Ah, ok. So if the big straw was built in the vacuum of space, and the capped end were inserted into the atmosphere, lowered to sea level, and then opened, air would rush in and fill up the straw up to the height that column of air would be if it were part of the atmosphere outside the straw but no more?

    While that would make an interesting satellite launching cannon, there goes my bond villainesque blackmail-the-world-for-a-billion-dollars device. :-(
  8. Sep 14, 2009 #7
    Yeah, exactly. In fact you can do this with a normal-sized straw. Put your thumb over the top so the air can't get out, and stick it in your drink. The inside of the straw stays dry. Then take your thumb off the top and water rushes in to fill the inside of the straw to the same level as the outside, but no higher.
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