What happens when you hurl your homework in the air?
This week Derek is with Professor Hugh Hunt from the
University of Cambridge and three student volunteers
from the Norwich School. They're going to be throwing
books into the air and learning about the science of
To do the experiment, you will need:
A rectangular (preferably hardback) book
An elastic band to hold the pages together
How to do the experiment:
1 - Take the book and put the elastic band around
it to keep the pages together.
2 - Hold the book in your hands so that the book is
flat and the spine is facing away from you.
3 - Throw the book up into the air and make it spin
so that the spine moves towards you and then away again.
Catch the book and see if it looks any different.
4 - Now hold the book so that it is flat between the
palms of your hands with the spine facing away from
you. Spin the book again, aiming to make the spine come
towards you and away again. Look again at the position
of the book once you've caught it. (Don't worry if you
can't see any difference in stages 3 and 4!)
5 - Now hold the book as though you were looking at
the cover ready to read it. The spine should be on the
left hand side. Throw it up into the air and make it
spin. What changes about the position of the book? How
has the cover changed relative to how it was before
you threw it?
What's going on?
In steps 3 and 4, nothing particularly remarkable
happens! They spin about the same axis all the way through
and land in your hands in the same orientation.
However, when you throw it in step 5, it should have
landed in pretty much the same way but rotated round
180 degrees. You will have noticed this because the
front cover should be upside down. Anybody watching
you throw the book will see that it starts going up
normally and spinning but then does a flip when it gets
to the top.
So what's happening? When you throw it up into the
air, the book starts spinning as you would like it to
spin, but it doesn't last long! It turns out that spin
about this particular axis is unstable, and doesn't
like to spin in the way you would expect for very long.
This makes the bus start to tumble out of control. Once
it's done this flip, it magically starts spinning nicely
again, but the other way round. This means that when
you finally catch the book, it's lying in your hands
This is all rather different to the other two ways
of spinning the book. These two spin directions are
what we call stable. You can think of this by imagining
holding a pencil by its tip. If you hold the pencil
so that the rubber is pointing downwards, it won't move.
In fact, you could hold it like that for hours because
gravity is forcing downwards and the direction is stable.
In contrast, if you try to balance a pencil on your
finger tip with the rubber pointing upwards, the direction
is unstable and the pencil falls down.
You can imagine that this pencil is a little bit like
a pendulum. When it's swinging backwards and forwards
down the bottom, then it's stable situation. But if
you imagine tipping it right up around 180 degrees,
it wouldn't stay there for very long. It would swing
down, go right the way round and come back to the top
again. So if it fell to the right, it comes back again
from the left. Instabilities quite often involve moving
away from where you started and coming back again the
other way round. This is why the book turns 180 degrees
from its original position. However, if you throw it
again, the flip turns the book in the opposite direction
and brings you back to the beginning.
If you toss the book up high enough, it might do two
flips. This will make it come back into your hands with
the cover in the same orientation as before you threw
These same spinning stabilities and instabilities can
be seen in objects such as mobile phones because they're
the same shape as books (one long axis, one medium axis
and one short axis). Even a cat falling out of a tree
uses the same principles to move itself around.
Want to find out more?
There are hundreds of other interesting facts and a
whole host of science behind spinning objects. To find
out more and to see video footage of some similar experiments,
you can go to Hugh