Name: Stephanie M.
How do yo-yos work? Why do they come back up after being
thrown down? Is there a physical explination?
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
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
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.
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.
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Update: June 2012