Exerting Force: The Strength of Air Pressure
 

	Fun Experiment to Try at Home!
		Purpose To test the strength of air pressure.   Materials - pencil - two 3-ounce (90-milliliter) paper cups - 12-inch (30-centimeter) piece of string - suction cup with hooks (of the type used to secure hanging crafts to windows) - tap water - 100 or more pennies (metal washers will also work)   Procedure 1.  Use the pencil to make holes on opposite sides of one of the cups just beneath its rim. 2. Thread the string through the holes and tie the ends to form a loop. 3. Fill the second cup about half full with water. 4. Dampen the suction cup with water by dipping it into the cup of water. 5. Press the suction cup against a flat, smooth, vertical surface such as the front of a metal filing cabinet. 6. Hang the empty cup by placing the string over the hook of the suction cup. 7. Add 5 to 10 coins at a time to the cup. Coins or other things can be added to the cup until the suction cups pulls away from the surface.   Sum It Up! Pressing the suction cup against the surface forces the air out of the underside of the suction cup. The water on the suction cup helps to form a seal around its edges to prevent air from entering. The air in the room pushes and holds the flattened suction cup against the surface, causing it to stick.

The force of the air pushing on your body is stronger than the force of two teams of horses.
 
In 1657, Otto von Guericke, the mayor of the German town of Magdeburg, demonstrated how strong air pressure is. He did this by fitting together two copper bowls to make a hollow sphere. He then pumped the air out of the sphere, forming a vacuum. A perfect vacuum contains nothing, not even a single atom. So the hollow sphere was most likely not a perfect vacuum, but it was very close to empty, which made it a partial vacuum.

There was very little air inside the sphere to push the bowls apart, and the greater push of the air on the outside held the bowls together. A team of horses was attached to each side of the sphere; the number of horses is usually shown as four per team. Try as they might, the horses were not able to pull the bowls apart. Air pressure is very strong. It does not crush our bodies because our bodies push back with an equal force.

For more information about forces, see Janice VanCleave's 203 Icy, Freezing, Frosty, Cool & Wild Experiments." (New York: Wiley, 1999)