Physics Van Outreach Show

Physics Van - Bernoulli Ball

Air

Materials:

Bernoulli Equipment />
<ul>
  <li>Leaf blower</li>
  <li>NERF soccer ball/inflatable beach ball</li>
  <li>Extension cord</li>
  <li>Toilet paper attachment for leaf blower</li>
  <li>Toilet paper</li>
  <li>Sheet of loose-leaf paper</li>
</ul>

<h3>Key Points:</h3>

<ul>
  <li>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.  </li>
  <li>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.</li>
  <li>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.</li>
  <li>This effect is a part of what makes airplanes fly, and many schools use this
    effect to explain flight.  While the Bernoulli Effect <i>is</i> real and
    it <i>does</i> 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.</li>
</ul>

<h3>Warnings:</h3>
<ul>
  <li>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.</li>
  <li>Another volunteer should hold the leaf blower steady while the presenter balances
    the ball.</li>
  <li>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.</li>
  <li>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.</li>
  <li>It’s easy to accidentally turn on the leaf blower, so leave it unplugged when it’s not in use.</li>
  <li>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.</li>
</ul>


<h3>Things to talk about:</h3>
<ul>
  <li>The Bernoulli Effect was first studied by Daniel Bernoulli.</li>
  <li>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.</li>
  <li>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.</li>
  <li>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.</li>
  <li>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.</li>
  <li>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!</li>
</ul>

<p>Click here for the <a href=Adobe PDF version of this demo (text only), for printing purposes.