Need a quick thought provoking demonstration that inspires a conversation in your classroom? Perhaps another activity to demonstrate air pressure that will enrich your weather or physics unit?
All you’ll need is a wooden spool, an index card, and a thumbtack. Take a deep breath and exhale slowly to show students the power of air or let Dr. Boyd F Edwards demonstrate it for you!
Trim a 3” x 5” index card in half to create a 3” x 2.5” card.
Push a tack into the center of the card.
Place the point of the tack into the tube in the center of a wooden thread spool. The tack keeps the card centered in the stream of air.
Face downwards as if you’re looking down at the floor. Hold the spool in one hand while supporting the card with the tack in your other hand. Blow gently and steadily down through the core of the spool and drop your hand away from the card.
Students see that the card does not fall!
What happened to the card? Why doesn’t it fall? How does gravity pull on the card? How does the air push on the card? How could blowing air into the spool change the air pressure?
EXPLAIN that by blowing into the hole of the spool, the pressure between the spool and the card drops. The stationary air in the room is still pushing up on the card from below. The stationary air holds the card in place while you blow on the spool. When you stop blowing into the spool, the air pressure equalizes. Gravity causes the card to fall.
At the beginning of the demonstration the air (shown by arrows)
pushes up and down on the card so you have to hold it in place
or gravity will pull it to the floor.
During the demonstration the air pushes up on the card
but not down on the card. Since air pushes up with more force
than gravity pulls down on the card it does not fall to the floor.
What if we use a larger or smaller card? What if we try it without the thumbtack? What about trying it with a pushpin? Does it matter if the card is replaced by copy paper or heavier cardboard? What if I don’t lean forward so that I am facing downwards at the same angle?
Want to have students try it but you don’t have enough spools? I would cover the top of the spool with wide piece clear tape that extends beyond the edge of the spool and then a punch a hole in the tape with a sharp pencil. After several students have tried the activity, I pull off the stack of tape and start over! Often students would bring in extras spools from home I could keep and they were so small storage was not a problem.