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Name:  Stephanie M.
Status:  student
Age:  18
Location: N/A
Country: N/A
Date: 2000-2001


Question:
How do yo-yos work? Why do they come back up after being thrown down? Is there a physical explination?


Replies:
Stephanie,

There's less to this than meets the eye. I assume you are seeking information on the kind of Yo-Yos that are used to do tricks.

That kind of Yo-Yo is just an odd-shaped dumbbell with a string looped around its axle. Examine a Yo-Yo and you will see that the string is not fastened to the axle -- just looped around it. When you send the Yo-Yo on its downward path, the string unwinds to the bottom of the loop where the Yo-Yo continues to spin (the loop slipping on the axle) until it stops.

However, if you give the Yo-Yo a quick jerk when it reaches the end of the looped string, the increased friction between the string and the axle causes the string to get a "bite" on the axle that is of sufficient strength that it begins to climb the string using the energy of rotation stored in the spinning mass.

It is possible to make a Yo-Yo with a string and axle which are so smooth that it cannot climb back up the string. No one wants one of those.

Regards,
ProfHoff


While the string is tightly wrapped around the yoyo's axle, it cannot fall without unwinding, so the rate at which it gains downward momentum is constrained to agree with the rate at which it gains angular momentum. When the yoyo reaches the end of the string, things get really ineresting. If there is enough friction between the string and the axle to maintain the constraint coupling rotation with translation (i.e., up/down motion) then the yoyo's angular momentum will cause it to climb the string. If there isn't enough friction, the yoyo will "sleep" at the bottom, spinning without climbing. The frictional force between the string and the axle depends on the force pressing them together, and the person using the yoyo can control this. If she makes a soft landing, so there is no jerk at the bottom, the axle is likely to slip, and you get a sleeper. If there is a strong enough jerk, it will increase the frictional force between the string and the axle enough to cause the yoyo to climb.

Tim Mooney



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