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HOW COME? Yo-yo is energy in motion

How does a yo-yo work? asks a reader.Can you Hop the Fence, Milk the Cow, Ride the Horse, and then Walk the Dog -- without ever setting foot on a farm? If so, you're probably familiar with yo-yo tricks.

A simple wooden yo-yo, physicists say, illustrates many of the discoveries of science. Case in point: A fundamental law of the universe is that energy can't be created or destroyed (scientists call it the "conservation of energy.") Energy already exists, but its form can change in an instant.

Imagine water behind a dam. When the dam is closed and the water is waiting, it has potential energy. Open the dam, and the water cascades down, pulled by the force of gravity. Suddenly, the water's potential energy has been transformed into kinetic energy -- energy of motion.

Rushing against the blades of a turbine, the water's kinetic energy powers the turbine, which is connected to an electrical generation system. So the water's kinetic energy has been changed into electrical energy. And so on, into your home -- where the electrical energy is transformed into heat energy, warming your house.

Something similar happens with a yo-yo. A yo-yo drops, and its potential energy changes to kinetic energy. But unlike water falling over a dam, a yo-yo rotates as it drops, its rate depending on the thickness of the string bundle wound around its middle.

As the yo-yo falls, its string unspooling, it spins faster.

Some of the energy that would go into falling has been transformed into the rotational energy of spinning. So about halfway through the fall, the yo-yo's descent speed actually slows.

At the end of the string, the yo-yo bounces. The string begins to rewrap around the shaft, yo-yo still spinning. If the bounce it felt wasn't hard enough, the yo-yo will "sleep," according to physicist Jearl Walker, of Cleveland State University. To wake it up, he says, jerk back on the string. If there is enough friction between the loops of rewrapping string, the yo-yo will keep turning and rising.

Back at the top, its kinetic energy transformed into gravitational potential energy, the yo-yo drops again. But because of friction, a yo-yo's kinetic energy gradually changes into heat energy in the string and spool. Fail to tug up on the string each time, and the yo-yo loses more and more momentum, finally coming to a full stop at the bottom.

How big can yo-yos get and still work? In 2010, a 1,600-pound yo-yo broke the Guinness world record.

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