Although we can”t see it, we live at the bottom of an ocean of air! This ocean is about 300 miles high, and all that air presses down on us and everything around us. Why aren”t we crushed under the pressure and weight of all this air? Because there is also air inside of us and under us pushing upward and outward with the same pressure. This experiment allows us to see that air pressure does exist.
Here’s what to do”
- Fill a glass with water right to the top.
- Slide a piece of cardboard over the top of the glass, making sure that no air bubbles are left in it.
- Hold the cardboard tight against the glass as you turn it upside down over a bucket or basin.
- Take your hand away from the cardboard.
Here’s what happens”
There should be no spillage of water, as air pressure pushing up on the cardboard keeps it in place. The air pressure is greater than the weight of the water pushing down on the cardboard from inside. We can continue to hold the water in the glass this way until the cardboard gets wet and starts to sag. At this point, air will start to leak into the glass and travels to the top of the glass. When the air pressure inside the glass (on top) becomes equal to the air pressure outside the glass (on the bottom), the earth’s gravity then pulls all the water out of the glass.
Were you successful?