The Duncan Yo-Yo celebrates its 75th anniversary in 2004. To a historian, the yo-yo is the marketing success story of Donald Duncan. To a child (of all ages) it is a challenging toy. "To a physicist, however, a yo-yo is a remarkably fun example of a flywheel," said Gordon Jones, assistant professor of physics at Hamilton College.
"Flywheels, used in fuel efficient cars to cutting-edge fusion research, are simply ways to store energy in a spinning disk. For such a simple toy, the yo-yo stores energy in spinning motion in a remarkably clever way."
Jones began thinking about the physics of the yo-yo when his daughter, Charlotte, acquired the toy. He noted, "Yo-yos actually slow down as they near the end of their fall. As the yo-yo approaches the bottom, the wrapped string gets closer and closer to the center of the yo-yo. The result is making the disk spin faster and fall slower as it reaches the end of the string. Rather than wasting energy jerking your finger," said Jones, "a well designed yo-yo puts almost all of its energy into storage just before it reaches the end of the string. That makes for less finger pain and more time 'sleeping.' Many tricks involve manipulating the yo-yo while it appears to sleep. At the end of the trick the yo-yo jumps back onto the hand using the stored energy," he explained.
The stability of a yo-yo during tricks is due to its angular momentum. This is the same effect that causes footballs, bullets, and the earth to stay pointed in the same direction. "Just as a tight spiral keeps a football from twisting in flight," Jones said, "the spin of a yo-yo keeps it facing the same direction as it gets flung and flipped. Stability is convenient for snapping the toy back onto your hand at the end of a trick, but it is essential for balancing the yo-yo on its string and other tricks."
"The yo-yo is a pretty cool toy from a physics point of view," Jones added. "If I ever teach a mechanics class I will definitely make the students solve for its motion."
The American yo-yo craze was sparked by a Philippine bellboy named Pedro Flores who attracted crowds with his yo-yo demonstrations. In 1929, Flores' fledgling company was bought by Donald Duncan who brought the yo-yo into the national spotlight through demonstrations, contests, and a clever marketing collaboration with William Randolph Hearst's newspaper empire.
