Up and down
Question: How does a yo-yo work? What makes it come back up the string and
how do people perfect the technique of 鈥渨alking the dog鈥�? Some experts can make
the yo-yo start to climb up the string by holding the string between their
finger and thumb, and hitting this hand with their spare hand. Even if they then
let the string go, the yo-yo still neatly winds it all up.
How do they do this?
Answer: If you examine a modern yo-yo, it consists of two heavy discs that
are connected by a smooth axle. The string is not tied to the axle but is simply
looped around it.
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The string can be visualised as a U starting at your finger, going down
around the yo-yo鈥檚 axle and back again to your finger. The string is twisted,
forming a loop at the bottom, so that the yo-yo can鈥檛 jump out of this 鈥淯鈥�.
As the yo-yo is released from your hand, the initial potential energy it has
is converted into angular kinetic energy. It stores most of this energy, like a
gyroscope, in its spin or angular momentum. Good yo-yos have more mass on the
outside of the disc than on the inside, in order to keep the yo-yo light while
at the same time maximising angular momentum.
The end of the string loops loosely around the axle of the yo-yo, leaving the
yo-yo free to spin inside the loop when it reaches the bottom of its descent.
The friction between the string and the axle here is not great enough to allow
the axle to grab the string and start winding itself up.
However, if you give a small jerk to the string鈥攖his happens in normal
yo-yo usage when the string jerks as the yo-yo reaches the bottom of its
descent鈥攚hich momentarily reduces the string鈥檚 tension. The friction of
the string against the axle is then enough to allow the string to start wrapping
itself around the axle.
Once the string has made one turn around the axle, the loop starts to act as
if it is attached to the axle鈥攖he outer turns of string stop the inner
loop from slipping any more and the yo-yo then begins to climb up the string,
converting its kinetic energy back into potential energy.
The sensitivity of the yo-yo to remain spinning depends on the friction
between the axle and the string. This can be adjusted by twisting the string to
make it tighter or looser around the axle, or by waxing the inner axle to
lubricate it. The smoother and looser it is, the easier it is to do tricks, but
the harder it is to make the yo-yo climb back up the string.
Jim Thompson
Jakarta, Indonesia
Answer: The string is the most important part of a correctly set up yo-yo. It
should be a single piece of string looped in half and twisted together, not tied on as so many often are
(see Diagram). The twist should be unravelled a little
on the spindle鈥攊t should not be too tight because this looseness is what
allows yo-yo experts to carry out all their various tricks.
Simon Scoltock
Bristol
Answer: 鈥淪leeping鈥� a yo-yo involves dropping the yo-yo down the string, but
not pulling it back up so that the yo-yo spins freely, remaining at the bottom
of its cycle. The key to sleeping is to have a loose loop around the yo-yo
spindle.
A tight loop will cause the string to grab the spindle, making the yo-yo
travel back up after reaching the bottom of its cycle. Having a loose loop
allows the yo-yo to spin freely because the spindle isn鈥檛 tightly held by the
loop.
To 鈥渨alk the dog鈥�, simply sleep the yo-yo (a good hard throw will put lots of
energy into the spin) and allow the spinning yo-yo to touch the ground. It will
walk itself.
To 鈥渟pank the baby鈥�, sleep
the yo-yo and slap the back of the string-holding hand after a sufficient
amount of time. By slapping the back of your hand you jar the string, and if the
loop is not too big it will, through increased friction, catch the spindle and
the yo-yo will return.
Yali Friedman
Toronto, Canada
Light鈥檚 out
Question: My son has 鈥済low-in-the-dark鈥� plastic stars on his wall. With the
lights out, I shone a pen-sized laser pointer on one, expecting a brighter spot
to appear on the luminous plastic, but it darkened instead.
It was actually possible to draw dark lines over the surface of the plastic
with the laser. Can anyone explain how this happens?
Answer: This is a nice manifestation of everyday quantum mechanics. In this
context light behaves as a stream of particles (photons, or light quanta), each
having a characteristic energy, which depends on the wavelength of the
light.
In the visible range, red light has the lowest photon energy, violet light
the highest.
The electrons in an atom (or ion or molecule) may adopt different states,
each with a particular energy. Usually the electrons are found in a state of
lowest possible energy. This condition is known as the ground state.
When light strikes the atom, its electrons may become 鈥渆xcited鈥� to states of
higher energy by absorbing photons of suitable energy. In most cases the excited
electrons quickly return to the ground state, discharging their excess energy as
light or heat.
The special thing about 鈥済low-in-the-dark鈥� materials is that the excited
electrons can stay stuck in a longer-lived state, called a metastable state,
which can survive for minutes or even hours.
Still, every second, a proportion of the excited electrons return to the
ground state, producing a (usually greenish) afterglow. This glow persists for
as long as there are electrons left in these excited states and this is what
gives them their special property.
The point now is that the photon energy of the red light used in laser
pointers is too small to bring any electrons from the ground state up the
metastable state.
However, the light is able to kick electrons from the metastable state up to
slightly higher states, from which they rapidly fall back down to the ground
state.
So, the intense red light shone onto the 鈥済low-in-the-dark鈥� material
discharges the stored energy, while green or blue light would, instead, add to
the stored energy.
Hans Starnberg
Department of Physics
University of Gothenburg, Sweden
This week鈥檚 question
Unblemished reputation: What makes stainless steel stainless?
Catherine Read
Farnborough, Hampshire