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