Ball – Bernoulli’s Principle (Weekly Unplugged Project)

By , January 26, 2009 11:48 am

The theme for this week’s Unplugged Project was ball.  My original thought was golf ball.  We live on a golf course so our backyard provides us with a constant supply of golf balls!

We are still feeling very “sciency,” so I decided to stick with a science project like last week.  For a while now, I have been wanting to try a trick that I have seen on a larger scale at a science museum:  “floating” a ball in a stream of air to demonstrate Bernoulli’s Principle.

As a pilot and a flight instructor, I am very eager for my children to understand the physics of flight.  I have given them many lessons on the shape of airplane wings.  Each time we go someplace in our plane, I ask them to tell me about how the shape of the wing creates the lift that makes the plane fly.  In fact they have heard me go on about it so many times that they are now at the eye rolling, “here goes Mom again” stage.

I thought that they would enjoy this “magic trick” and felt they were ready for it as an added lesson relating to the airplane.

All you need is a hair dryer and a ping pong ball.  We had no ping pong ball (and golf balls are too heavy to use with the hairdryer) but I found a lightweight plastic ball in the playroom.  It was larger than a ping pong ball, but weighed about the same.

Turn the hair dryer on to high (if you have a “cool” setting, that’ll save your fingers from burning as you play with the ball) and point it straight up toward the ceiling.  Place the ball in the air flow.  If your ball is light enough, it should hover there.

You can slowly and gently tilt the hair dryer sideways and the ball will “follow,” remaining in the air stream until the angle is such that the force of gravity is stronger than the “lift” generated.  The ball will then fall to the ground.

For more fun, use a shop vac and some heavier balls.  (I recommend that you do this outside!)  Remove the hose from the vacuum port and attach it to the exhaust opening.  It will now blow air instead of suck it in.  Golf balls will work.  Try the lightweight ping pong ball that you used with the dryer too and you’ll see that with the stronger airflow, it will balance much higher.

Why do balls “float” this way?  Because of Bernoulli’s Principle!  Bernoulli’s Principle basically says that the faster a fluid (or air) flows, the less pressure it exerts.

To understand this experiment, you also need to know that air flowing over a curved surface flows faster than air flowing over a straight surface (the reason for this is complicated, but has to do with the same mass of air being forced through a smaller area – the curve takes up more space than the straight edge).

So:  The air that flows over the curved surface of the ball must flow faster than the air that goes straight up around the ball without touching it.  The faster flowing air in contact with the ball exerts less pressure than the surrounding air that is traveling straight up.  The lower pressure ball is “trapped” inside a cylinder of higher pressure and is thus held in place.

How does this relate to airplanes?  An airplane wing is curved on the top, and fairly flat on the bottom, as you can see in this drawing:

The air flowing over the upper curved surface flows faster than the air flowing along the lower, straighter surface. This means that the pressure on the top of the wing is less than that below the wing. Thus the wing is “lifted” or sucked upwards.

Airflow over a wing:

After the fun of last week’s video science lesson, we made another one this week. It is a bit longer (nearly 3 minutes), but we hope you enjoy it!

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If you did a ball Unplugged Project this week, then please link to your project itself (rather than just your blog) in the Mr. Linky below.  If you didn’t join us, but would like to find out more about it, please don’t link, but read more here.

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The theme for next week’s Unplugged Project will be:

Color

Enjoy!

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10 Responses to “Ball – Bernoulli’s Principle (Weekly Unplugged Project)”

  1. Meg says:

    So, we have this toy that looks just like your science project! I’m quite sure you don’t own it, because it fits in the “loud toys” category…it plays random songs while you play with it. (BTW, we like this toy, but we too have TOO many toys that play songs when you bump into them or look at them sideways.)

    Anyway, this toy is like an upside down hair dryer and it blows lightweight balls into the air and sometimes they fly away, sometimes they drop into a little chute and go back down into the air stream. I never even thought to use it for teaching, but I will now!

    Megs last blog post..Unplugged – Painting With Marbles

  2. Wishy says:

    This is such a cool post! I’m gonna do this one with my Kiddo! She’ll have fun and learn about science all at the same time! Thanks! (It looks fun for me too. I really learned something today!)

    Wishys last blog post..I’m a sucker for cute animal stories

  3. KateinNJ says:

    Hi, very cool..we’re in this week.
    I just added the pictures.

  4. kristen says:

    Awesome science project!! We did juggling balls http://pepperpaints.com/2009/01/26/juggling-balls/

  5. […] 27, 2009 in letter B, unplug your kids | Tags: balls, unplug your kids This weeks unplug your kids was Balls I knew we had to do it cuz it was a Bb word and we love […]

  6. Dayna says:

    Love this. I look forward to doing some more science type projects with my little one as she gets older. My hairdryer just broke – and only lets out cold air now…may be the perfect time to give this one a try!

    Daynas last blog post..Unplugged Challenge – Ball

  7. as you can tell we are into the crafty over here. We are having a blast with your weekly unplugged project.

    Jazzcreationss last blog post..Eco-Friendly Tip of the Day

  8. […] Unplugged Project theme is introduced and it’s up to you how to interpret it.  For example, the most recent theme was ball, so the blog author taught her children all about Bernoulli’s Principle using a hair dryer!  […]

  9. jimmy101 says:

    Excellent home experiement!

    One thought though, I don’t believe your explanation of how a wing provides lift is correct. The Bernoulli principle does contribute to lift but it isn’t the most important affect. Indeed, if Bernoulli was all that matters then it would be impossible for a plane to fly upside down. Most planes can fly upside down (as long as the airframe can handle an upside down wing load, but that has nothing to do with lift). Even big planes like a 707 have been flown upside down. In addition, planes with equal upper and lower chords have been flown (the Wright Flyer had equal wing chords as do paper airplanes).

    Most of the lift provided by the wing is due to the deflection of air downards by the wing. Air forced down means the wing is forced up. As long as the leading edge of the wing is above the trailing edge the air flow is forced downwards, regardless of the top and bottom chord lengths of the wing. In your airflow over airfoil drawing the leading edge is above the trailing edge. That tilt (pitch) is providing more lift than the Bernoulli affect.

    With a enough information on the flight envelope of a particular aircraft the relative contribution of wing pitch and the Bernoulli affect can be estimated. What is the level flight pitch when the plane is right-side up versus upside down? If Bernoulli isn’t contributing then the pitch is the same. If Bernoulli contributes then the pitches are different and the difference in pitch is related to twice the Bernoulli affect.

  10. Mom Unplugged says:

    Hi Jimmy,

    Thanks for your excellent point! I believe that you are correct. The Bernoulli principle is only a piece of the explanation of how a wing produces lift. The Newtonian explanation that you describe is also a big part of it. Lift certainly does vary with the angle of attack of the wing, increasing as the angle of attack increases, up to the critical angle of attack and the resulting stall. As you mention, this is what makes inverted flight and “flat” wings, or even sails possible.

    You make a great point, however I am not convinced that the angle of attack piece of the equation is more important than Bernouilli. The relative importance of Newton vs. Bernoulli seems to be a matter of intense debate, even among physicists and aeronautical engineers. If you have never Googled “newton bernoulli lift” I recommend that you try it, especially if this interests you (I find it quite fascinating!) Try reading this site: http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html, or this forum for an example of heated debate: http://www.pprune.org/tech-log/355523-why-do-aircraft-fy-flat-plate-lift-vs-bernoulli.html. Here is a very extensive article on lift and airfoils: http://www.av8n.com/how/htm/airfoils.html#sec-bernoulli. I believe both theories play a part in lift-producing airfoils. I’ll leave the proportion-determining to those who know more than I!

    PS. I am so glad you brought this up, I wondered if anyone would. I intentionally kept my post very, very simple and that bothered me. But I feared glazing the eyes of those readers who are less enthusiastic about about aviation and science than I am (which would probably be most of them). Thanks again for the great comment!

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