Yo-yos are one of the most popular toys around, even after hundreds of years.
On the surface, the yo-yo is an incredibly simple toy -- it's really nothing but a spool attached to a length of string. But in the right hands, it can be something extraordinary: An accomplished yo-yoist can send the yo-yo flying out in all directions, make it hover in mid air, then snap it back into his or her palm. Ordinary string and wood (or plastic) are brought to life!

This may seem like magic, but it's actually just physics at work. Both the classic yo-yo and the sophisticated "automatic" yo-yos that have sprung up in the past few years are remarkable demonstrations of fundamental scientific principles.

Now, we'll examine these principles to find out what makes yo-yos behave in such an unexpected way. We'll also look at the history of yo-yos and see how they've changed through the years.

One Good Turn
The yo-yo is one of the most popular and enduring toys of all time. The ancient Greeks were playing with them more than 2,500 years ago, and there's some evidence that the Chinese had developed similar toys before that. In any case, the yo-yo has demonstrated phenomenal longevity -- it's older than any other toy except the doll.

There have been several variations on the yo-yo design through the years. In the original design, which was still popular until the early 20th century, the string was tied securely to the axle. This design achieved huge popularity in Europe in the 18th and 19th century, where it had a number of names, including bandelore, quiz and L'emigrette.

In the modern yo-yo, brought to the United States from the Philippines in the 1920s (see below), the string is only looped around the axle. To understand the significance of this difference, let's examine the physical principles at work in both sorts of yo-yos.

In the original yo-yo design, the string was secured to the axle. In the modern design, the string is only looped around the axle, allowing the yo-yo to "sleep."

In both designs, the yo-yoist winds the string tightly around the axle. Sitting in the yo-yoist's palm, the yo-yo has a certain amount of potential energy (energy of position). This potential energy takes two different forms:

When the yo-yo is released, both forms of potential energy change to kinetic energy. The yo-yo spool falls straight to the ground, which builds a certain amount of linear momentum (momentum in a straight line). At the same time, the string unwinds, and the spool spins, which builds angular momentum (momentum of rotation).

When the yo-yo reaches the end of the string, it can't fall any farther. But, because it has a good deal of angular momentum, it will keep spinning.

The spinning motion gives the yo-yo gyroscopic stability. A spinning object resists changes to its axis of rotation because an applied force moves along with the object itself. If you push on a point at the top of a spinning wheel, for example, that point moves around to the front of the wheel while it is still feeling the force you applied. As the point of force keeps moving, it ends up applying force on opposite ends of the wheel -- the force balances itself out. This phenomenon keeps a yo-yo's axis perpendicular to the string, as long as the yo-yo is spinning fast enough. (See How Gyroscopes Work to learn more.)

If the string is attached securely to the axle, as in the original design, the spinning axle will grip the string and start rewinding it; the yo-yo will travel back up the string. The yo-yoist must give a slight tug on the string as the yo-yo rewinds, in order to compensate for the energy lost to friction.

In the modern yo-yo, there is less friction between the string and the axle, since the string is only looped around the axle. When the spool completely unwinds, it will not automatically grip the spring -- it will simply spin freely. To get the yo-yo to rewind, the yo-yoist jerks on the string a little bit. This tug briefly increases the friction between the string and the axle so that the axle starts rewinding the string. Once it starts rewinding, this sort of yo-yo will return to the yo-yoist just like the older design.

The ability to make the yo-yo spool spin on the end of its string, or "sleep," made yo-yoing a much more interesting challenge. Yo-yoists try to keep the spool sleeping while making shapes with the string and swinging the yo-yo around them. Another trick is to "walk the dog" -- let the spinning spool roll along the ground before pulling it back in.

Over the years, manufacturers have come up with a number of mechanisms to make it easier to do these sorts of tricks. In the next section, we'll check out a few of the more popular variations found in modern yo-yos.

Name Dropping
The word "yo-yo" and the modern yo-yo design come from the Philippines. Unlike the original Chinese and Greek yo-yos, Filipino yo-yos (the word means "come come" or "come back" in the native Tagalog language) had the ability to "sleep." This unique toy may have been an adaptation of the Chinese yo-yo, or it may have developed out of a Filipino hunting weapon. In any case, it dates back at least a few hundred years in the region.

In the 1920s, a Filipino immigrant named Pedro Flores decided to bring this yo-yo design to the United States. He achieved some success right away, and in 1929, he sold his company to a businessman named Donald Duncan. Duncan trademarked the name "yo-yo" and, over the next few decades, built his company up into the premiere yo-yo manufacturer.

Duncan's competitors released similar products, under several different names (including "twirler" and "whirl-a-gig"). But the public adopted the term "yo-yo," leading the rival companies to challenge Duncan's trademark. In 1965, the Federal Court of Appeals ruled that the term had become generic, and so could be used by anyone. In the same year, the Duncan company went bankrupt, selling the Duncan name to the Flambeau Plastics Company, which still sells Duncan-brand yo-yos today.

Asleep at the Wheel
For most people, the hardest part of yo-yoing is getting the spool to sleep long enough to pull off some tricks. To get an ordinary yo-yo to sleep for a while, you have to throw it with a lot of force so it builds up strong angular momentum. But when you throw a yo-yo fast, your hand tends to jerk, pulling the spool back in. Beginning yo-yoists also have trouble "waking" a yo-yo (pulling it out of a sleep). It takes a lot of practice to get the right balance to put the yo-yo to sleep successfully.

Yo-yo manufacturers have tried a number of things to make it easier to keep a yo-yo sleeping, and to make it wake up again. One of the simplest improvements was to redistribute the weight in the yo-yo, in order alter its moment of inertia.

An object's moment of inertia is a measure of how resistant it is to changes in rotation. This is determined by two factors: how much mass the object has and how far that mass is from the object's axis of rotation. Increasing mass makes an object harder to rotate and harder to stop rotating, as does increasing the distance between the mass and the axis of rotation (a rolled-out slab of clay, for example, is harder to spin than a tight clay ball with the same mass).

If you increase the moment of inertia in a yo-yo's discs, the yo-yo will be able to sleep longer; it takes more work to stop the rotation. For this reason, manufacturers often concentrate the weight in high-performance yo-yos around the outer edge of the spool. Since the distance is larger between the axis of rotation and much of the mass, the spool will have a greater moment of inertia.

Another approach is to further reduce friction between the yo-yo string and the axis. One popular method is to configure a ball bearing assembly around the yo-yo axle, so the axle itself is separated from the string. You can see how a typical bearing system works in the diagram below.

A yo-yo design with a ball bearing arrangement: Ball bearings reduce friction between the string and the axle, making it easier for the yo-yo to "sleep."

The bearing assembly consists of two races, essentially grooved tracks for ball bearings. The inner race immediately surrounds the axle, and the outer race is spaced a bearing's width apart. The ball bearings are positioned between the two races. The yo-yo string is looped around the outer race, so it never touches the axle itself. The races are not bound together: The inner race can tilt slightly inside the outer race.

When you throw the yo-yo, the unwinding action spins the outer race. The force of the throw tilts the inner race inside the outer race, which increases the friction between both races and the ball bearings. Effectively, the tilting action locks the races together, so they turn in unison. In this way, the spinning outer race spins the inner race, which spins the yo-yo axle.

When the yo-yo reaches the end of its string, the gyroscopic motion of the spinning discs tends to level the races out, so they are aligned with one another. With this configuration, the ball bearings can move smoothly between the two races. If the bearings are properly lubricated, they will significantly reduce friction between the two races. This reduced friction lets the axle spin more easily, which increases sleeping time.

To wake the yo-yo, you jerk on the string. This tilts the outer race in relation to the inner race, increasing the friction on the bearings. The bearing assembly locks up again and the spinning motion of the outer race carries the yo-yo back up the string.

This mechanism makes it easier to keep a yo-yo sleeping, but it doesn't help much with waking the yo-yo up. In the next section, we'll look inside the new "automatic" yo-yos that sleep and return on their own.

Pop the Clutch
In the yo-yo craze of the 1990s, a new sort of automatic yo-yo started popping up everywhere.
Yomega, the leading manufacturer of these yo-yos, advertised their model as "the yo-yo with a brain." It does seem like these yo-yos have some level of intelligence, since they know exactly when to sleep and wake up, but the "brain" is actually just a centrifugal clutch. You can see how this mechanism works in the diagram below.

In this yo-yo, a clutch mechanism releases the axle when the yo-yo is spinning quickly and grips it again when the yo-yo slows down. This makes the yo-yo come back automatically before it slows to a stop.

As in the ball-bearing yo-yo we looked at in the last section, this yo-yo design does not let the string touch the axle directly. Instead, the string is wound around a spindle piece. The axle, which is mounted to the two halves of the yo-yo, runs through the middle of the spindle, but the two pieces are not actually connected.

The spindle and axle will move in unison when the yo-yo spins slowly, however, thanks to the yo-yo's clutch mechanism. The clutch mechanism, which is housed inside one of the yo-yo discs, consists of two metal spring-loaded arms. These arms are weighted at one end and connected to the body of the yo-yo at the other end. When the yo-yo is stationary or spinning slowly, the springs press the arms up against the spindle, so the spindle's rotation turns the entire yo-yo. But as the yo-yo speeds up, centrifugal force pushes the weighted ends of the arms outward, against the springs. The arms release the spindle, so that the spindle and the rest of the yo-yo move independently.

When you throw the yo-yo, it initally spins slowly. The clutch is locked, and the discs are spun by the unwinding spindle. But just before the yo-yo has reached the end of its string, it is spinning fast enough that the clutch releases the spindle. The disc's angular momentum keeps the yo-yo spinning, but the spindle slows down. Eventually, the discs slow down too, and the centrifugal force acting on the arms decreases. When the outward centrifugal force dips below the inward force of the springs, the arms clamp shut on the spindle. This transfers the spinning motion of the discs back to the spindle, which causes the spindle to rewind the string and return to your palm.

This toy is a lot more elaborate than the terra-cotta yo-yos of ancient Greece, but it has the same basic appeal. Yo-yos continue to be so popular because of their wonderful simplicity. There's some undefinable magic about taking an ordinary spool and, with nothing but a flick of the wrist, turning it into an active, spinning top. No matter what advanced mechanisms are added to yo-yos, this simple joy will be the heart of their appeal.

To learn more about yo-yos, including how to pull off some popular yo-yo tricks, check out the links on the next page!

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