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Homework Help: The Enchanted Cardboard Experiment

  1. Feb 22, 2006 #1
    The "Enchanted" Cardboard Experiment

    Fill the drinking glass with water up to the brim.

    Place the cardboard on top of the glass and hold it down. Some extra water may escape.

    Hold the cardboard against the glass with your fingers and turn the glass over.

    Carefully remove your fingers.

    You should be surprised and amazed by this strange occurrence.

    Repeat the experiment with a new piece of cardboard and only one inch of water in the glass.

    Could someone explain to me why the water stays in the glass? I don't have to know the answer to this question just yet but I read two different explanations that are probably saying the same thing...but they sound different to me.:rofl:

    Ex. #1: The water stays in the glass because the pressure of the air outside the glass is greater than the pressure of the water against the cardboard.

    Ex. #2: When you turned the glass upside-down, a small amount of water dribbled out without any air bubbling back in. Gravity was pulling down on the water and the water was pulling down on the air inside the glass. But being a gas, the air inside the glass spread out more. That meant the air molecules inside the glass weren't pressing against each other as hard as they were before. The air outside the glass, on the other hand, was pressing in on the cardboard just as hard as it ever was. And that difference in air pressure was enough to actually suspend the water in mid-air, with nothing more than air pressure keeping it from escaping.
  2. jcsd
  3. Nov 9, 2016 #2


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    This is where we part company; blotter paper has usually been used, and the reduction in pressure isn't ordinarily adequate to keep the glass "sealed."
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