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Air pressure problem (gr.10)

  1. Sep 24, 2005 #1
    hi!

    i kinda grasp the concept of atmospheric pressure and how it affects everyday stuff... but i'm stuck with this one problem that i can't seem to understand.

    The experiment involves filling a cup with water. Then, a cardboard piece is placed completely over the opening of the cup and the cup is inverted (with the cardboard held in place over the opening). As you may know, the cardboard becomes 'suctioned' to the inverted cup. I just don't understand WHY????

    i know it has something to do with atmospheric pressure. My sketchy hypothesis is that pressure inside near the top of the inverted cup is lower than that of the pressure at the inside bottom of the cup. owing to the fact that air travels from an area of high pressure to an area of low pressure, i am *thinking* that the atmospheric pressure outside the cup is striving to get to that low pressure area inside the the cup thus, causing suction.

    I would really, REALLY, REALLY appreiciate if someone could explain to me why this happens or to confirm whether or not my hypothesis is correct. Thanks a bunch :-)
     
  2. jcsd
  3. Sep 24, 2005 #2
    There is no suction inside the cup, only the weight of the water. The cardboard is held in place by the force of the atmospheric pressure pushing up on it. Since atmospheric pressure is about 14.7 pounds per square inch, it easily overcomes the weight of the water and holds the cardboard in place.

    Hint: Don't use the term suction in front of a science teacher when referring to atmospheric pressure and its effects. They just hate it when you do that. :smile: Even a suction cup is actually held in place by the pressure of the atmosphere pushing on the "outside" of the cup.
     
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