5 Ways to Demonstrate Air Pressure to Children
Nov 22, 2010
by Aurora Lipper Mechanical Engineering Magazine
This story was updated on 10/11/2022.
Note: when conducting at-home science experiments with children, an adult should always be present. Even the simplest experiments have the potential to go wrong.
The ordinary pressure of the air surrounding us is 14.7 pounds per square inch—but this can change based on a few factors, such as when the wind blows or a car or airplane accelerates. Wherever the air pressure is higher, there will be a stronger force or push against an object. Similarly, when an air particle speeds up, it actually “pushes” less.
Imagine that fast-moving air particles are in so much of a hurry that they don’t have time to apply force—this is the principle used to make airplanes fly. When a plane moves along the runway, the air above the wing speeds up, lowering the pressure so that the air below the wing can push the plane upward.
Interested in testing out these principles in a more tangible way? Try one or more of the following experiments:
Water Glass Trick
Step 1: Fill a cup one-third with water.
Step 2: Cover the entire mouth of the cup with an index card.
Step 3: Holding the card in place, take the cup to the sink and turn it upside down.
Step 4: Remove your hand from underneath.
Voilà! Because the water inside the cup is lighter than the air outside, the card is held in place. This is due to about 15 pounds of force from the air pushing up, while the force of the water pushing down is only about one pound of force.
Step 1: Fill a 2-liter soda bottle half full of water.
Step 2: Take a long straw and insert it into the mouth of the bottle.
Step 3: Wrap a lump of clay around the straw to form a seal.
Step 4: Blow hard into the straw—then stand back.
When you blow into the straw, you’re increasing the air pressure inside the sealed bottle. This higher pressure pushes on the water, forcing it up and out of the straw.
Step 1: Put a ping-pong ball inside the wide part of a funnel.
Step 2: Blow hard into the narrow end of the funnel.
Step 3: You’ll notice that the ball doesn’t pop out of the funnel—but why?
This is because as you blow into the funnel, the air moves faster and lowers the air pressure underneath the ball. Because the air pressure is higher above the ball than below it, it’s pushed down into the funnel—no matter how hard you blow or in which direction you point the funnel.
The Million Dollar Bet
Step 1: Place an empty water or soda bottle down horizontally on a table.
Step 2: Roll a piece of paper towel into a small ball about half the size of the bottle opening.
Step 3: Tell a friend you’ll pay them one million dollars if they can blow the ball into the bottle.
Don’t worry about losing money—because this is impossible. No matter how hard someone tries to force more air into the bottle, there's no room for it. The air will flow right out, pushing away the paper ball.
Step 1: Blow up two balloons and attach a piece of string to each.
Step 2: Place one balloon in each hand, holding them by the string.
Step 3: Position the two balloons so they are at your nose level and six inches apart.
Step 4: Blow hard into the space between the balloons.
As you lower the air pressure in that space between the balloons, the pressure of the surrounding air becomes higher. This automatically pushes the balloons together, causing them to “kiss.”
[Adapted from “Top Ten Air Pressure Experiments to Mystify Your Kids-Using Stuff From Around the House,” by Aurora Lipper, for Mechanical Engineering, January 2008.] Read More: How to Mentor Young Engineers Experiential Learning and Cooperative Education Pay Off Engineering Education, Family Style