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Project Summary

Difficulty	6
Time required	Average (about one week)
Prerequisites	None
Material Availability	Readily available
Cost	Very Low (under $20)
Safety	When practicing with the yo-yo, keep it away from breakable objects. Be aware of where other people are so that nobody gets hit with the yo-yo.

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Objective

To investigate how string length affects yo-yo sleep time.

Introduction

Most people are familiar with the yo-yo as a fun and cool toy. It's fun learning to make it work properly and cool to learn different tricks to perform with it. But did you know that the yo-yo is an ancient toy? It is thought that the yo-yo originated in China more than 2,500 years ago! The first historical reference to the yo-yo was in 500 BC. A Greek vase from that time shows a painting of a Greek boy playing with a yo-yo. You can see for yourself in Figure 1, below. The next historical reference to the yo-yo as a toy came in 1765 on a box made in India. The miniature box was hand-painted with an image depicting a girl playing with her yo-yo. Within 25 years, the yo-yo made its way from India to the upper classes in Scotland and France, and then into England. The first mention of the yo-yo in America was in 1866 in a patent for an improved design. However, the yo-yo finally caught on in the United States in the 1920s. It was then that a Filipino immigrant, Pedro Flores, brought over the "Filipino" yo-yo, an even further-developed design. Instead of the yo-yo string being tied directly to the axle between the two discs, the string is tied in a loop around the axle. See the HowStuffWorks online article, referenced in the Bibliography below, for a visual in the "One Good Turn" section. This innovation allows for longer sleep times and thus, more complicated tricks. This is because there is less friction between the string and the axle than when the string was tied directly to the axle. In the late 1920s, Pedro Flores sold his yo-yo company to Donald F. Duncan, who helped to popularize the yo-yo to its present-day status.

Figure 1. This Greek vase depicts a boy playing with a yo-yo. (Wikipedia, 2008.)

Click here to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org

The yo-yo might seem like magic, but it is just physics at work. There are three parts to the basic yo-yo: the two disks and the axle that joins them. When the string is wound around the axle and the yo-yo is sitting in the yo-yoist's hand, it has two kinds of potential energy. Potential energy is the energy stored in an object. For example, if you hold a ball straight out in front of you, ready to drop it, it now has potential energy. The first kind of potential energy in the yo-yo is the potential to fall to the ground. Since the string is wound around the axle, the second kind of potential energy is the potential to spin. When the yo-yo is thrown, the potential energy is converted into kinetic energy. Kinetic energy is the energy of motion. In the example of the ball held in front of you, once you let go of the ball and it is moving through the air, it has kinetic energy. The potential energy in the ball changes to kinetic energy as soon as you let go of it. As the yo-yo falls, the string unwinds and the yo-yo spins. The yo-yo is building angular momentum. As the string unwinds, it generates torque on the axle and this torque is responsible for the yo-yo's angular momentum. When the yo-yo reaches the end of the string, it can fall no farther, but it can keep spinning because its angular momentum remains constant. It is in this state that the yo-yo can "sleep." When a yo-yo sleeps, it continues to spin at the end of its string. This is an example of gyroscopic stability, which is the resistance of a spinning object to a change in its axis of rotation. Instead of just leaning over and stopping, the yo-yo continues to spin and looks as if it is floating. Eventually, friction stops the yo-yo from sleeping. If there were no friction, the yo-yo would continue to spin indefinitely. If you want to stop the "sleep the yo-yo" trick, just give the yo-yo a small tug. The string should catch the axle and the yo-yo should rewind. Click on the DragonflyTV link to watch John, Kevin, and Minna investigate the physics of the yo-yo.

Now it's your turn. See if you can confirm what John, Kevin, and Minna found out about how the length of the string affects a yo-yo's sleeping time. Let's hope that your sleepy yo-yo gets enough rest!

Terms, Concepts and Questions to Start Background Research

• Friction
• Potential energy
• Kinetic energy
• Angular momentum
• Torque
• Gyroscopic stability
• Axis of rotation
• Centrifugal clutch

Questions

• Why does the yo-yo rewind?
• Why does the yo-yo look like it is floating at the end of the string?
• What are the two kinds of potential energy of a yo-yo that is held in the player's hand?

Bibliography

• TPT. (2006). Yo-Yos by John, Kevin and Minna. DragonflyTV, Twin Cities Public Television. Retrieved July 30, 2008, from http://pbskids.org/dragonflytv/show/yoyos.html

The following websites have good information on yo-yos.

• Harris, T. (2001, December 5). How Yo-yos Work. HowStuffWorks, Inc. Retrieved July 29, 2008, from http://entertainment.howstuffworks.com/yo-yo.htm
• Krantz, V. (1997, September). History of the Yo-Yo. American YO-YO Association Newsletter. Retrieved July 29, 2008, from http://www.ayya.net/articles/history.html
• Wikipedia Contributors. (2008, July 30). Yo-Yo. Wikipedia: The Free Encyclopedia. Retrieved July 30, 2008, from http://en.wikipedia.org/w/index.php?title=Yo-yo&oldid=228822814

For help in creating graphs try the following website:

• National Center for Education Statistics. (n.d.). Create a Graph. Retrieved June 30, 2008, from http://nces.ed.gov/nceskids/CreateAGraph/default.aspx

Materials and Equipment

• Basic butterfly-style yo-yo (3); available at toy stores
• Yardstick
• Extra yo-yo string; available at toy stores
• Scissors
• Location where you can play with a yo-yo on a long string without it getting tangled up
• Stopwatch
• Volunteer to help time the trials
• Lab notebook
• Optional: Books from your local library or bookstore about yo-yos and yo-yo tricks

Experimental Procedure

1. To start this science fair project, you should watch the DragonflyTV video in the Introduction. You can also take a look at books about yo-yos to learn how to yo-yo.
2. Open one of the yo-yos and get familiar with it.
3. Using the yardstick, measure from your belly button to the ground and remember this number. Now, unwind the yo-yo all the way. Cut the yo-yo string to the length you measured with the yardstick. The distance between your belly button and the yo-yo on the ground should be the same distance that you measured with the yardstick. Play with the yo-yo so that you know how to get the yo-yo to sleep—be sure to practice in an area away from breakable objects and where you won't hit anyone.
4. Open the second yo-yo and cut the string 12 inches shorter than the first yo-yo.
5. Open the third yo-yo. The string length of the third yoyo should be 12 inches longer than the string length of the first yoyo. If the string length of the third yoyo is longer than required, then cut off the excess. If the string length is shorter than required, tie on some extra string to the end. If you are tying on extra string, make sure that you have 1 inch extra to account for tying the knot. Re-measure after you've tied your knot to make sure it is 12 inches longer than the first yo-yo.
6. Now use the first yo-yo and have the volunteer use the stopwatch to time how long you can get the yo-yo to sleep. The volunteer should begin timing when your yo-yo hits its end. Record the time in a data table, like the one shown below, in your lab notebook.
7. Repeat step 6, with the same yo-yo, two more times, recording each sleep time in your lab notebook.
8. Repeat steps 6-7 using the second yo-yo with the shorter string. Record all data in your lab notebook.
9. Repeat steps 6-7 using the third yo-yo with the longer string. For this test, be sure to go to a spot where you can throw the yo-yo without it hitting the ground, getting tangled, or hitting someone. Record all data in your lab notebook.
10. What are your observations? Which string produced the longer sleep time? Why do you think so? Plot your data on a scatter plot so that you can show your results to other people. If you'd like to create your graph online, try a site like Create a Graph. Label the x-axis String length and the y-axis Sleep time. You can plot the individual sleep times, along with the average sleep times.

Yo-yo	Yo-yo Sleep Time
1	Trial 1
	Trial 2
	Trial 3
	Average
2	Trial 1
	Trial 2
	Trial 3
	Average
3	Trial 1
	Trial 2
	Trial 3
	Average

Variations

• Try replicating this experiment with a fancier yo-yo, like a Yomega®. A Yomega yo-yo has a centrifugal clutch that is designed to make yo-yos sleep longer.
• Investigate the physics of simple toys like the yo-yo. Replicate this experiment using tops. You can make your own top by using heavy papers and a pencil. Vary the length of the pencils or the size of the papers and see which design produces a top that spins the longest.
• Grab a book about yo-yos and check out a few more tricks. Create an investigation similar to the DragonflyTV kid scientists', but with a different trick and variable. Does a heavier yo-yo help you "walk the dog" longer? Are plastic yo-yos better than wood ones when it comes to "going around the world?"
• Compare dropping a ball out a window with dropping a yo-yo (with equally long strings, but different sizes) out a window and time how long each takes to come down. For the yo-yo, less of the potential energy would be converted into kinetic energy because some of the potential energy would be used in spinning the yo-yo (some of the potential energy would be converted into angular momentum). What do you predict will happen?

• For more science project ideas in this area of science, see Physics Project Ideas.

Credits

Michelle Maranowski, PhD, Science Buddies

• Yomega® is a registered trademark of Yomega, Corp.

Last edit date: 2008-10-22 16:00:00

Career Focus

If you like this project, you might want to think about career opportunities in Physics.

Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To reach that goal, they observe and measure natural events seen on Earth and in the universe, and then develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest atomic particles. Their theories are then applied to human-scale projects to bring people new technologies, like computers, lasers, and fusion energy. Learn more about this career: Physicist.