Hey. Friction, right? It, or a lack of it, is a big part of what makes motion possible. A car couldn’t roll if there wasn’t enough friction for the tires to stick to the road somewhat; without friction, the wheels could turn all they wanted, but the tires would just slide across on the surface of the street. Even airplanes need friction to move, and they drive on the air. Kind of. Just go with it, okay? Jeez.
If you want to demonstrate how friction and air combine to help aeroplanes fly, just try this relatively simple Demo Science science experiment. Even the dumbest of dumb little kids in our class is sure to be mesmerized. (If you can stop picking his nose long enough to pay attention, Kevin!)
Bottoms Up, Coins Down
For this experiment, you’ll need a glass beverage receptacle (the thicker the lip, the better), a penny, and a table. Drain the glass before the kids show up, then set it right side up on the table. Balance your penny flat on the rim of the glass. This might be the most difficult part, depending on what was in the glass when you started.
Gather your students ‘round to observe, then blow on the coin from the side. Just a little puff of breath should be enough to make it fall into the glass, which is the intended result. It’s not much for the little snots to observe, but it sets up the next part, so it is important to the experiment.
Balance your penny on the rim of the glass again, just as you did the first time around. And again, watch closely, students. This time, stand back a bit (about a foot or so), and blow on the coin in a more powerful and controlled fashion. You should be aiming directly for the edge of the coin, and trying to avoid the air hitting the top or bottom of it. If executed properly, the penny should skip across the opening of the glass and land on the far side. (Maybe practice this beforehand a few times.)
So… That’s Science How?
The first time, the penny fell into the glass because you didn’t blow on it with enough force to overcome the friction between the two. There was enough oomph to move it, but not enough to totally break it free of the friction.
When you blew on the penny the second time, the greater force and speed of the air were able to overcome friction and briefly lift the coin into the air. There, the only friction the penny had to deal with was that caused by the air around it, which is very little friction at all. Without gravity, or anything around it for the coin to run into, it could have carried on on its flight path for much longer.
If this version goes well, you may want to try it again with other, larger coins and possibly a different glass. Ask those smelly students of yours what they think will happen if you use the same blowing force on, say, a quarter. Or if you try the same technique to blow a penny across a wider container/opening. The possibilities are endless!*
* Not really, but there are quite a few potential options.