The science of yo-yos
by Chris Woodford. Last updated: July 30, 2012.
Who'd have thought you could have so much fun with a bit of plastic on the end of a string? If you think nothing could be simpler than a yo-yo, it's time you tried looking into the science behind it! How does it keep spinning so long? How can it "sleep" (hang at the end of the string)—and what makes it climb back up again? Why do yo-yos feel so strangely stable as they spin? There's a lot of physics going on in your yo-yo. Let's take a closer look!
Photos: Yo-yo skills in action. Photo by Eric Harris courtesy of US Air Force and Defense Imagery.
What makes yo-yos go up and down?
A yo-yo might look like a toy, but it's also an energy converting machine. Understanding how it constantly changes energy from one kind into another is the key to figuring out how it works. If you're not familiar with scientific terms like potential energy and kinetic energy, you might want to browse through our energy article before you go any further.
When you hold a yo-yo in your hand, it has potential energy: it stores energy because it's high above the floor. When you release it, the potential energy is gradually converted into kinetic energy (the energy something has because it's moving). When a yo-yo is spinning at the bottom of its string, virtually all the potential energy it had originally has been converted into kinetic energy. As a yo-yo climbs up and down its string, it is constantly exchanging potential and kinetic energy—much like a rollercoaster car.
Photos: The string is like a yo-yo's fuel tank: it supplies the energy the yo-yo needs to keep moving. Photo by Michael Larson courtesy of US Air Force and Defense Imagery.
A spinning yo-yo actually has two different kinds of kinetic energy: one kind because it's moving up and down the string and another kind because it's spinning around. When you release the yo-yo from your hand, it falls toward the ground just like a stone, and it picks up speed because it's falling. But a yo-yo is different from a stone because it has string wrapped around its axle. As it falls, it starts to spin. That's why a yo-yo falls much more slowly than a stone: some of the energy that should be making it fall quickly is actually being used to make it spin around at the same time.
Whatever it's doing, and wherever it is on the string, a yo-yo usually has a mixture of three different kinds of energy:
- Potential energy—because it's a certain height above the floor.
- Kinetic energy of movement—because it's moving up or down relative to the floor.
- Kinetic energy of rotation—because it's spinning around.
In a perfect world, a yo-yo could rise and fall on its string forever. But as the string spins on the plastic axle, friction (the rubbing force between two things that are in contact and moving past one another) uses up some of its energy. Although you can't see it happening, the spinning yo-yo wheels also rub against the air that surrounds them. This air resistance also eats away at the yo-yo's energy supply. If you don't keep giving the yo-yo more energy, by pumping the string up and down, it slows down very quickly and grinds to a halt. Every time you tug the string, you jerk the yo-yo so it keeps on spinning. In effect, you are recharging its energy batteries with each tug.
Artwork: A yo-yo starts off in your hand with only potential energy. Once it's spinning and moving, rising and falling, it has a mixture of potential energy and two kinds of kinetic energy. You have to keep adding more energy by jerking the string.
Why does a spinning yo-yo feel weird in your hand?
Things that are moving like to carry on moving. We call this phenomenon momentum (loosely speaking, momentum means "mass in motion"; things have momentum because they have both mass and velocity). A truck speeding down the freeway has more momentum than a car going the same speed because it has more mass. A person has more momentum when they ride a bicycle than when they walk because a bicycle goes faster (it has a higher velocity or speed).
Just as a yo-yo has two kinds of kinetic energy, so it has two kinds of momentum: linear momentum (because it moves in a straight line, up and down on the string) and angular momentum (because it spins around). All spinning objects have angular momentum. And anything that's spinning around likes to keep on spinning so its angular momentum stays the same. If you try to make it spin a different way, it will compensate by changing its motion somehow. If an ice skater is spinning in a circle with her arms out and she suddenly brings them in, she'll spin much faster than she did before. That's because she changes the way her mass is distributed. Her body compensates for this by changing her velocity to keep her angular momentum constant.
Angular momentum is why a gyroscope behaves so strangely. A gyroscope is a heavy wheel mounted on a framework of three axles that allow it to spin around in three dimensions. Once you set a gyroscope spinning, it will strongly resist any attempts to make it spin another way. So if you try to tilt it, it will tilt back the other way. Like an ice skater, it tries to keep its angular momentum constant or, as physicists say, to conserve its angular momentum. A spinning yo-yo behaves just like a gyroscope. That's why it feels strangely stable as it spins on the string. It feels almost as though it has a built in stubbornness to change its movement. That's one reason why you can do all kinds of neat tricks with it!
Artwork: A spinning yo-yo behaves a bit like a gyroscope. This still image comes from Wikicommons, where you can find a superb animated version of the same figure.