How can a yo-yo fall and climb the same string?

A simple yo-yo can roll down its string and climb back toward a hand. It does not get a fresh supply of energy at the bottom. Much of the energy that brings it back was stored when it was held high, then carried in its spinning motion as it fell.

Consider a yo-yo whose string is attached to its narrow axle. Gravity pulls the yo-yo down, while the string pulls up on the axle. Because the string pulls to one side of the axle’s center, it also gives the yo-yo a turning push. As the string unwinds, the toy moves downward and spins faster. Energy stored by its raised position becomes energy of movement and rotation.

Near the bottom, the string is almost unwound, but the yo-yo is still spinning. It can keep turning so the string winds onto the axle again. Winding shortens the free length of string and draws the yo-yo upward. The toy gives up spinning energy as it gains height. It need not reverse its spin just because it reverses its up-and-down travel.

An ordinary yo-yo does not return to exactly the same height forever. Rubbing parts, air resistance and the change at the bottom transfer some energy into warming and sound. A player’s hand movements can add energy and keep the motion going. The toy’s repeated journey is an exchange of energy, not a machine making energy from nothing.

Some trick yo-yos have a bearing or a loose string loop that lets the body spin while it stays near the bottom. This is called sleeping. Their return mechanism differs from the simple attached-string example. The string’s connection to the axle changes what a spinning yo-yo can do.