water upside down

Have you ever knocked over a drinking glass? What happened to the water? With this experiment you can hold a drinking glass upside down, even if it is full of water.


  1. fill the glass to the rim with water
  2. put the postcard on top of the full glass, pictureside down
  3. press softly on the card to remove all air underneath
  4. turn the glass upside down, make sure no air gets in the glass
  5. hold the glass in one hand
  6. question 1: Does the water get out of the glass?

    quesiton 2: Would the experiment also work with a bigger glass?


question 1:
Most of the time you won't notice that air presses on all things, from all sides. While you are holding the glass upside down, the air presses on the card, upwards. The water inside the glass presses on the card by it's own weight, downwards. The water won't fall out of the glass because the air pressure is stronger than the force of the water.

question 2:
A bigger glass would probably work as good as a small one. The air pressure could even hold a bucket full of water. To make it actually work, you just have to make sure there is no air and no opening between the card and the water.


extra explanation

The air is pressing on the postcard as it presses on all things, at the rate of 1 kilo per square centimeter (1kg/cm2). One liter of water is approximately one kilo. Per square centimeter of the surface of the postcard, the air pressure can hold a liter of water. The drinking glass has a surface of at least 15 square centimeters, so it will be easy for the air pressure to hold the water on the other side of the card.

The postcard is needed because water is thin and fluid. For instance, crème fraîche is thicker and less fluid. As a result, it is easy to hold a cup of crème fraîche upside down without a postcard at all. In physics the thickness of a fluid is called viscosity. Crème fraîche has a large viscosity, so there is more friction between the molecules than in water. Water has a small viscosity so the water molecules will flow more easily. The postcard is there to hold them together.


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