# How Yo-Yo's Work

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The yo-yo is one of the oldest toys in existence. Despite its lasting popularity, it is one of the simplest toys around. The early versions of they toy were made simply from carved wood and a string. No one is quite sure of the origin of the yo-yo but many say China. Others say that ancient Greeks as far back as 2,500 years used yo-yos for entertainment. The motion and the process of playing with a yo-yo incorporates many concepts in physics of momentum, energy, friction, etc. With simply a spool and a bit of string a yo-yo is a fantastic model of physics hard at work.

In the first versions of the yo-yo, the string was tied directly to the spool, which made the yo-yo snap back up as soon as it was fully extended. In more modern versions of the yo-yo, the string is made into a loop around the spool, which allows for the yo-yo to "sleep" or keep spinning while fully extended. While the yo-yo is still in the user's hand, it has potential energy. It has the potential to fall, and also the potential to spin. Once the yo-yo is released, it falls towards the ground and converts its potential energy into kinetic energy. As the yo-yo falls, it builds up linear momentum and at the same time it builds up angular momentum as the string unwinds and the yo-yo spins. This builds up the yo-yo's rotational velocity so that when it sleeps, it can spin very quickly so that it can sleep for lengthy periods of time.

When the yo-yo's string is completely extended, the built up angular momentum combined with extremely little friction keeps the yo-yo spinning (in the case of the modern yo-yo that can sleep). This spinning of the yo-yo produces gyroscopic stability and the yo-yo's axis stays perpendicular to the string so long as the yo-yo is still spinning fast. When the yo-yo is sleeping, it is rotating away from the user. This means that the angular acceleration points toward the left based on the right hand rule. The concept of precession is also very important in yo-yos. If the yo-yoist throws the yo-yo down at slightly the wrong angle, the yo-yo will tilt to the left or right. The inner surface of the yo-yo will come into contact with the string and will rub the string, which will slow the yo-yo's spin. The yo-yo will then begin to rotate to the right or left depending on which way the yo-yo is tilted. If the yo-yo is tilted to the right and the string is rubbing on the left rim, the yo-yo will rotate to the left. If it is tilted to the right and the string is rubbing on the right rim, it will rotate to the right. This rotation is called precession. The contact between the rim and the string tries to tip the yo-yo back to the vertical position. At the same time, the angular momentum that the yo-yo has tries to keep the yo-yo spinning against the friction. The combination of these two cause the yo-yo to precess. This defines the behavior of a gyroscope. Precession also makes the direction of the angular acceleration of the yo-yo rotate along with the yo-yo.

In order for the yo-yo to "wake from its sleep" the user must give the string a gentle tug. This tug produces just barely enough friction between the string and the axle so that the string can rewind itself on the spool. The yo-yo will not make it all the way back up to the yo-yoists hand by itself due to frictional forces, so the yo-yoist needs to give the string an upward pull while the yo-yo is rewinding in order to give the yo-yo a little extra kinetic energy to compensate for friction. The yo-yo then goes back through its cycle except in reverse. The yo-yo converts its kinetic energy back into gravitation potential energy and when it stops in the hand of the yo-yoist, the yo-yo has solely potential energy again.

In the case of the original model of the yo-yo where the string is tied tightly to the axle, almost all of the physics that explains the use of a yo-yo is identical to the modern yo-yo except for the behavior of the yo-yo when the string is fully extended. The original yo-yo starts with potential energy. The potential energy is converted into kinetic energy as the yo-yo builds up linear and angular momentum. The next step is where the old yo-yo differs from the modern yo-yo. Because the string is tied directly to the axle, the friction is immediately great enough to cause the yo-yo's string to begin rewinding itself around the spool, whereas the modern yo-yo needs a small tug to momentarily create enough friction for the yo-yo to begin rewinding itself.

These physics concept have so far only described the simple up and down operation of the yo-yo, but many people use the yo-yo to perform all sorts of tricks. Many tricks involve keeping the yo-yo sleeping for lengthy periods of time. Many factors influence

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