Key concepts
Forces
Friction
Symmetry
Wind power
Introduction
Have you ever played with a toy pinwheel—or seen larger real-life versions, such as a windmill or wind turbine? Have you wondered why they look the way they do—with multiple blades, arranged symmetrically? Why can't they get away with just one blade? Or blades of different shapes and sizes? Try this fun project to find out!
Background
Spinning blades are used in a wide variety of machines. Some blades are built to harvest energy from the air, such as on windmills and turbines. The wind pushes on these blades making them spin, and then the energy from their movement is transferred to turn gears or generate electricity. There are also blades that use a powered motor to push on air—sometimes to move it (as with a fan) or to push an object through it (for instance, an airplane propeller).
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One thing all of these blades have in common is that they are installed in a symmetric pattern. This means they are equally spaced around the center, or "balanced." There might be two blades directly across from one another or four blades that form an "X" and meet at right angles in the middle. There could even be an odd number of blades (for example, three blades evenly spaced would be at 120-degree angles to one another). Why is it important for the blades to be symmetric? Why don't you see windmills with just one blade or with two blades on the same side? This project will show you the best way to get the blades spinning!
Materials
At least one pinwheel (that can be cut apart)
Hair dryer
Scissors
Adult supervision
Preparation
Gather all of your materials in an area where you can plug in and use the hair dryer. Have an adult operate the hair dryer about one meter away from where you will be standing with the pinwheel.
Procedure
Turn on the hair dryer.
Hold the pinwheel about a meter away from the hair dryer. Gradually move the pinwheel closer to the dryer until it begins spinning. How close does the pinwheel need to be before it starts spinning?
Now, have an adult help you use scissors to cut off one blade from the pinwheel as close to the base as you can. This makes the pinwheel asymmetric because the remaining blades are no longer evenly spaced around the center.
Again, hold the pinwheel about a meter in front of the hair dryer and gradually move it closer until it starts spinning. How close does the pinwheel have to be before it starts spinning this time?
Use scissors to cut another blade off the pinwheel. Depending on how many blades your pinwheel had initially, you will have to make a decision about which blade to cut and whether the pinwheel remains asymmetric or becomes symmetric again. For example, suppose your pinwheel had four blades to start. When you cut off one blade, it has three unevenly spaced blades, so it is asymmetric. Now you can cut off a second blade next to the one you just cut off, so the pinwheel will still be asymmetric, with two unevenly spaced blades. Or you could cut off a second blade across from the one you just cut off. This will make the pinwheel symmetric again, with two blades directly across from one another. If you have more than one pinwheel, you can try different combinations.
Hold in the pinwheel a meter in front of the hair dryer again and slowly move it closer. How close does the pinwheel need to be before it starts spinning this time?
Repeat this process until your pinwheel only has one blade left.
Does the number of blades affect how easily the pinwheel spins? Do you have to hold the pinwheel closer to the hair dryer sometimes?
Does how easily the pinwheel spins depend on whether the blades are symmetric or asymmetric?
Extra: Use a ruler or measuring tape to record how far the pinwheel needs to be from the hair dryer to start spinning in each trial. Then you can make a graph of your results, with the distance from the dryer versus the number of blades.
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Observations and results
You should have seen that it was "easiest" for the pinwheel to spin when it had a larger number of symmetric blades. As you removed blades, you might have needed to hold the pinwheel closer and closer to the hair dryer to make it spin. This is especially true if the blades were asymmetric. This can cause the blades to "wobble" and create more friction on the pinwheel's axle (the rod that supports the blades). Higher friction makes it harder for the blades to spin, meaning the wind must be stronger—so you have to hold the pinwheel even closer to the hair dryer to get it to spin. How do you think this affects engineers’ plans when they design and make wind turbines or airplane propellers?
More to explore
Spinning Your Wheels: Pinwheel Sensitivity, from Science Buddies
Wind Turbine Design for Optimum Energy Output, from The National High School Journal of Science
Strong Wind Science: The Power of a Pinwheel, from Scientific American
Wild Winds: Turbulent Flow around Structures, from Scientific American
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
