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Name: Kevin
Status: student
Grade: 9-12
Location: AL
Country: N/A
Date: 12/16/2005


Question:
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?


Replies:
Kevin,

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.

Dr. Mellendorf


Dear Kevin,

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 momentum.

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 momentum.

Greg Bradburn


Kevin,

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.

Ryan B.lscamper



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