Why does a yo-yo yo?S

Find out about the interplay of forces that make a yo-yo bounce.

Most of us have spent some time with the string of a yo-yo cutting off the circulation in one finger, trying to get a yo-yo to do tricks. Parents give yo-yos to their kids in fits of nostalgia, and end up regretting it when the toy invariably goes through a coffee table or window when the kids try to do something flashy.

The idea of a yo-yo is pretty simple one. Three forces are at work; friction, angular momentum, and gravity. Gravity is pretty much the engine of the yo-yo. When you release the yo-yo, or throw it down slightly, gravity pulls it on down to the ground. This builds up kinetic energy in any object, but gravity is not the only force that acts on the yo-yo. The string, wound around it, pulls it sideways. The combination of forces gets the yo-yo to spin.

This makes angular momentum build up in the yo-yo. Momentum is the tendency of an object to keep doing what it's doing, even when acted on by another force. Angular momentum is the tendency of an object to just keep a'spinning, even when a force holds it back.

It's when the yo-yo gets to the bottom of the string that its angular momentum is tested. A string is not tied around a yo-yo. It's actually looped around the middle axle of the yo-yo, and then each side of the loop is twisted together to form what looks like one unified string. When the yo-yo hits bottom, it's spinning in the cradle of this loop.

Why does a yo-yo yo?S

The string still exerts a frictional force on the yo-yo, but not enough to stop it, or to re-wind the string. As people know, the trick is to give a little tug on the yo-yo just as it hits the bottom of the string. Those of us who were, shall we say, differently coordinated, it was hard to get the timing right, and the yo-yo would just sit at the bottom of the string sullenly until it stopped spinning. That's the effect of just a little friction – gradual dissipation of angular momentum.

What you want instead is a short, sharp pull of friction against the bottom of the yo-yo, and a pull of the string does this. It forces the string against the bottom of the yo-yo, increasing the friction. The yo-yo can no long spin its little heart out against the string. If it wants to keep spinning, it has to take the string with it. It does so, and the string winds back up. Since the yo-yo has to pull against gravity, it gradually loses angular momentum and slows as it moves up.

Basically, the trick with a yo-yo is to work it so that the force of angular momentum and the friction at the bottom is strong enough to overcome the pull of gravity on its way back up. Don't ask me how "walking the dog" works, though. Never could manage that.

Via Physlink.