Water in an Inverted Cup

This demonstration can be modified for use as a magic trick.

Materials:

1. Glass of water
2. Piece of cardboard that is larger than the mouth of the glass.

Procedure:

1. Fill the glass up with water.
2. Place the piece of cardboard over the mouth of the glass.
3. Holding the cardboard against the mouth of the glass, invert the glass.
4. Release the hand slowly.

Explanation

Water can remain in an inverted glass with the piece of cardboard underneath because atmospheric pressure is acting upward on the cardboard, holding it up together with the water. There is little air pressure within the g;ass, so the downward force acting on the cardboard is mainly the weight of the water, which is to the order of several newtons whereas atmospheric pressure exert an upward force of several thousand newtons.

Modification:

1. Drill a small hole in a plastic cup, near the base.
2. Seal the hole with your thumb and fill the cup with water.
3. Place the cardboard over the mouth of the cup.
4. Invert the cup together with the cardboard, while keeping your thumb over the hole.
5. Using a magic word as the cue, shift your thumb slightly to allow a little air into the cup. This will cause the cardboard and water to fall. As the air pressure within the cup is equal to that of the atmosphere.