I am interested in a kind of toys named
"yo-yo". It is a ball fastened to our hand with a string which can
return after we throw it out. My puzzle is that how can the yo-yo
run down clockwise but back anti-clockwise?
Actually a yo-yo is a little more than that. A very important part of a
yo-yo's structure is the size of the bar around which the string winds. For
a yo-yo to work well, this must be very small relative to the size of the
entire toy. This results in a much faster spin. Also, this design has much
more "rotational inertia": it will keep spinning for a longer time.
Also, a yo-yo does not reverse its spin. It keeps spinning in the same
direction throughout the process. To test this, draw a curved arrow on your
yo-yo to show the way the string is wound. Drop the yo-yo and let it come
back up. When it gets back up, you will see that the string is wound
backwards from when you dropped it. If the yo-yo had reversed its spin at
the bottom, the string would still be wound in the original direction.
Your puzzle is easy to answer! The yo-yo does NOT change its direction of
rotation. In fact it is the angular momentum you impart to the yo-yo which
causes it first to rapidly unwind on the way down, possibly "sleep" at the
bottom as it rotates on a loop in the end of the string, and then, after a
slight jerk which causes the string to start winding on the rod through the
center of the yo-yo, to come back up.
It is necessary to give the yo-yo enough of a throw in the beginning to
overcome friction with the air and with the string against the side of the
slot in the yo-yo. Without friction, if you just let the yo-yo unwind by
itself, it would gather speed on the way down which it would then lose on
the way up arriving back in your hand with zero speed (as it had left).
This can be shown by conservation of energy and/or conservation of angular
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Think about what happens when the yo-yo reaches the end of the string. The
yo-yo continues turning in the same direction due to its rotational
You seem to have a working knowledge of the principle of Inertia, that is,
the lack of motivation for things to suddenly reverse their direction. The
Yo-Yo does not go down clockwise and return counter clockwise. It continues
to spin in the same direction when it is thrown towards the floor, and
climbs back up the other side of the string.
Click here to return to the Physics Archives
Update: June 2012