The card that defies gravity!


You will need a water glass (a plastic container will work just as well) and an index card big enough to completely cover the mouth of the container. Any lightweight cardboard cutout (e.g., from a cereal box or even a greeting card) can be used, in case you don't have an index card handy.

1. Fill the glass container with water, say about 2/3rds full.

2. Place the index card on top of the glass.

3. Hold the glass with one hand and place your other hand on top of the index card.

4. Quickly turn the glass over, so it is upside down, with your hand still holding the index card in place.

5. Now, take your hand (keeping the index card in place) away. If the index card could talk, it would probably ask -- "gravity? What gravity?"

water in glassupside downdoes not fall!

Explanation: Again it is the air pressure on one side that is critical to the index card not falling down. If the index card were to start falling and even a few drops of water were to come out, the amount of space inside the glass occupied by the air will increase. Since the amount of air does not change but the amount of space increases, the pressure of the inside the glass will decrease. Then, like in the ruler/newspaper activity, the differential air pressure keeps the index card from falling.

An additional activity: If you used a plastic container rather than a glass container, then you can play a trick. First make a small hole (you can use a nail to make a hole) somewhere near the bottom part of the glass. Repeat the steps above, but make sure that you have a finger closing the hole so that (1) water doesn't leak out initially when it is right side up and (2) air doesn't flow into the container as you turn it upside down. The index card will stay in place as before. Now remove  your finger from hole, exposing the air inside the container to the outside. Now the air pressure above the card (i.e., air inside the glass) and below the card (air outside) cancel each other out and the weight of the water will push the card down.

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