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Magic of a yo-yo

Question:

What is the physics magic in a toy like a yoyo?

Answer:

Basically a yo-yo transfers rotational energy to translational energy (both potential and kinetic) and vice versa. That probably doesn't sound very exciting but it is probably the shortest way of explaining its motion.

In a bit more detail when the yo-yo is 'falling' down the string it is spinning and also loosing potential energy. This is converted into the kinetic energy of rotation in the yo-yo and also into linear kinetic energy – which means that it moves faster as it gets lower.

Giving the yo-yo a sharp jerk near the bottom of its path injects a bit more kinetic energy into it and this make it rotate faster and so enables it to 'climb' up the string. If you didn't do this, the yo-yo would finally come to rest because of the friction between it and the string.

If the string is not tied tightly onto the yo-yo then the yo-yo can spin when the string is fully unwound without climbing the string. To make it move up you can jerk it – this would force it against part of the string and so increase the friction.

Shooting the yo-yo out straight horizontally is really only like throwing something - the string then keeps it in a more or less straight line path although it will always be pulled down by gravity.



 
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© Keith Gibbs 2020