The simplest yo-yo resembles a flattened spool of thread – two disks (yo-yo halves) connected at the center with an axis of a sufficiently large diameter. To an axis there is a rope attached, which wounds around an axis. In the initial position, the rope is wound on the axle of the toy, and the second end of the rope is attached to the ring, which a person puts on his finger.
Let`s imagine that a person with the yo-yo simply set the toy free with an end of the rope on his finger. Under action of gravity weighty spool falls down, unwinding the thread and quickening its own spin. At the lowest point, the potential energy of the descended yo-yo completely transforms into kinetic energy of rotation. Rotational motion gives the yo-yo gyroscopic stability, so under normal outward conditions, the toy always remains in the same plane, not allowing a thread to get tangled.
When the yo-yo reaches the lower end of the rope, it cannot fall further. However since it has accumulated sufficient turning moment, it continues rotating by inertia. In a simple embodiment, the toy is fixedly attached to the axis, and under the action of the inertia the spool again begins ascending along the thread by winding it onto the axis, but now in the opposite direction. So the action cycles until the person with the yo-yo stops playing or until the thread gets tangled.
Yo-yo construction. [Graphic illustration]. Yo-yo Discussion forum. Retrieved from http://www.google.com/imgres?espv=210&es_sm=122&biw=1366&bih=643&tbm=isch&tbnid=4GBSYqv2zDnUzM%3A&imgrefurl=http%3A%2F%2Fwww.yoyonation.net%2Ftalk%2Findex.php%3Ftopic%3D10632.0&docid=bsHVhSErCHWTGM&imgurl=http%3A%2F%2Fimg503.imageshack.us%2Fimg503%2F1609%2Fyoyojamgk8.jpg&w=980&h=609&ei=_7HoUvvKA8WThgfY04GYDw&zoom=1&ved=0CFMQhBwwAQ&iact=rc&dur=202&page=1&start=0&ndsp=15