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