Physics Van - Bernoulli Ball

Bernoulli Ball

Air

Materials:

Key Points:

Air that moves fast in one direction doesn’t push as hard in the other directions as air that’s standing still. (i.e. fast moving air in the x-direction exerts less pressure in the y and z-directions.) So let’s say you have a stream of air moving along... if you put an air pressure gauge next to it, it would read less pressure than one on the other side of the room.
Using your mouth to force air over the top of the sheet of paper lowers the air pressure above the paper compared to the unmoving air underneath. The difference in pressure forces the sheet of paper up.
Using the leaf blower to force air over the top of the ball lowers the air pressure above the ball compared to the unmoving air underneath. The still air underneath the ball pushing upwards is enough to counteract the downward pull of gravity.
This effect is a part of what makes airplanes fly, and many schools use this effect to explain flight. While the Bernoulli Effect is real and it does play a role for certain types of planes, in general it’s a fairly small contribution, and for many planes it isn’t even necessary.

Warnings:

It takes some practice to learn how to balance the ball under the air stream. Be sure to practice it before attempting the demo during a show.
Another volunteer should hold the leaf blower steady while the presenter balances the ball.
The leaf blower is quite loud and is difficult to be heard over. It’s usually easiest to show the audience that the ball floats first and then explain the physics afterwards or vice-versa.
While the leaf blower is not dangerously loud, you may want to warn the audience before using it that it will make noise. Children with certain disorders may have severe reactions to loud noises. It’s best to check with the school beforehand to see if this will be a consideration.
It’s easy to accidentally turn on the leaf blower, so leave it unplugged when it’s not in use.
You may be questioned about this demo by adult audience members after a show! People who have read certain “myths of science” articles like to talk about how the Bernoulli Effect is wrong. In truth, the effect itself is perfectly real (and it does explain why the ball stays up in this demo!), but the way it’s explained in schools in relation to airplanes isn’t always right.

Things to talk about:

The Bernoulli Effect was first studied by Daniel Bernoulli.
The main point of the demonstration is that air moving fast in one direction pushes less (in any other direction) than air that isn’t moving.
But be careful; telling kids that fast moving air pushes less than unmoving air contradicts their experience (the wind moves fast and pushes hard). Make sure to stress that fast moving air pushes less perpendicular to the direction it’s moving, only DON’T use the word “perpendicular.” Instead, use your arms or whatever else you need to get your point across.
Demonstrate this point using the sheet of paper. Explain that blowing over the paper causes it to float because the unmoving air under the sheet pushes up more than the fast moving air on top of the sheet pushes down.
Have another volunteer help you demonstrate the effect using the leaf blower and the ball, but remember to warn the audience that it will be noisy!! Emphasize that this is the same as what just happened with the paper.
Have another volunteer bring up the toilet paper attachment (preferably already loaded with a full roll of paper) and fit it onto the end of the leaf blower. Cover the audience in toilet paper!

Click here for the Adobe PDF version of this demo (text only), for printing purposes.