Spinning Symmetry with Pinwheels
Have you ever played with a toy pinwheel, or seen larger real-life versions like a windmill or wind turbine? Have you wondered why all the blades are symmetric? Why can’t they get away with just one blade? Try this project to find out!
Spinning blades are used in a wide variety of machines. There are blades that harvest energy from the air, like on windmills and wind turbines. The wind pushes on those blades, making them spin. There are blades that use a power source, like electricity or a motor, to push on the air – sometimes to move the air (like a fan), or sometimes to move an object through the air (like a propeller on a plane or helicopter).
One thing all of these blades have in common is that they are symmetric. This means they are equally spaced around the center, or “balanced.” There might be two blades directly across from each other, or four blades that form a “cross” 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 each other). Why is it important for the blades to be symmetric? Why don’t you see windmills with just one blade, or two blades on the same side? This project will help you understand.
- At least one pinwheel
- Hair dryer
- Adult supervision for using the hair dryer
- Turn on the hair dryer.
- Hold the pinwheel about a meter away from the hair dryer. Gradually move it closer to the hair dryer until it starts spinning (if your arms aren’t long enough to hold both at once, have one person hold the hair dryer while the other person holds the pinwheel). How close does the pinwheel have to be before it starts spinning?
- Now, use scissors to cut one blade off of the pinwheel. This makes the pinwheel asymmetric because the remaining blades are no longer evenly spaced around the center.
- Hold the pinwheel about a meter in front of the hair dryer again, 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 4 blades to start. When you cut off one blade, it has 3 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 2 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 2 blades directly across from each other. If you have more than one pinwheel, you can try different combinations.
- Hold in the pinwheel in front of the hair dryer again and see how close it has to be to start spinning.
- Keep repeating 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 has to be from the hair dryer for each trial. Then you can make a graph of your results, with the distance from the hair dryer vs. number of blades.
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 closer to the hair dryer to get it to spin.
Ask an Expert
- Science Activities for All Ages!, from Science Buddies
- Spinning Your Wheels: Pinwheel Sensitivity, from Science Buddies