[Leigh Metcalfe]

Leigh Metcalfe asked the Naked Scientists:
   
According to the "Unspillable Water Experiment"

Air pressure can be stronger than gravity. This unspillable water experiment demonstrates its strength as it keeps the contents of a water glass in place, even upside down. (experiment follows after my question)

If that is the case, why does the index card fall when there is only air in the glass but stick when there is water in the glass? Is it really suction or some other property of the water that is holding the card in place?


What You'll Need:

    * Juice glass
    * Water
    * 4x6-inch index card

Step 1: Fill a juice glass full of water. Let the water run over so that the lip of the glass is wet. Be sure that you fill the glass right up to the top.

Step 2: Place a 4x6-inch index card on top of the full glass of water. Be sure to press the card down securely with your hand so that it makes a good seal all around the wet lip of the glass.

Step 3: Working over a sink, hold the card in place with one hand as you turn over the glass. Carefully let go of the index card. The card will stay in place, and the water will stay in the glass.

What happened? The force of air pressure against the card is stronger than the force of gravity on the water. The air pressure holds the card in place..

What do you think?

[daveshorts]

exactly the same way as the waterproof hankey kitchen science
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/waterproof-hankey-1/

The air is much easier to stretch than water, so the weight of the water in the glass can stretch the air enough to make enough space between the card and the glass at the bottom for air to leak in, at which point you are going to get wet