Why Toast Lands Jelly-side Downby Robert Ehrlich, Princeton
University Press, 拢12.95, ISBN 0 691 02891 5
YOU have just poured yourself a cup of coffee when the phone rings. What鈥檚
the best way of making sure your coffee is still hot when you鈥檝e finished? Put
milk in now, or when you鈥檝e finished on the phone?
Questions like this have rattled round common rooms in schools and
universities for donkey鈥檚 years. Now noted American science educator Robert
Ehrlich has assembled some into an intriguing collection of physics
demonstrations subtitled 鈥淶en and the Art of Physics Demonstrations鈥�.
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Ehrlich鈥檚 avowed aim is to give teachers a way of showing that physics is not
all particle accelerators and gravity wave detectors, but a science that affects
everyday objects in often surprising ways. Drawing heavily on papers from the
Physics Teacher and the American Journal of Physics, he
provides more than 100 demonstrations of physics concepts, ranging from Newton鈥檚
laws of motion to special relativity. Remarkably, the most sophisticated thing
he ever recommends using is an overhead projector.
So inspired was I by this 鈥渟mall science鈥� approach that I found myself doing
something I haven鈥檛 done in years: rolling up my sleeves and messing around with
rulers, water, taps and the like trying out Ehrlich鈥檚 recipes. How could anyone
resist testing his assertion that an egg floating in a glass of water rises when
placed under a running tap? Surely codswallop, you think: the blast of water
hitting the egg鈥檚 pointy bit must force it down into the water.
No, says Ehrlich: the streamlined shape of the egg means that the water鈥檚
momentum is little changed by striking it, and so by Newton鈥檚 second law of
motion there is little downward force on the egg. But as the stream strikes the
surface of the water in the glass it loses momentum: the resulting force
displaces a lot of water in the glass, and forces the egg upwards.
Amazing鈥攐r, it should have been. First problem: none of the farm-fresh
eggs in my kitchen would float. Clearly American hens are either fed on helium,
or lay stale eggs.
I eventually found an egg that hovered just above the base of a tumbler and,
sticking it under the tap, it reared up, just as Ehrlich had predicted. But the
cause seemed more likely to be the turbulent rebound of water off the bottom of
the glass, rather than anything to do with the surface.
To find out, I put the egg in a much deeper glass鈥攐nly to watch it sink
like a brick and stay resolutely near the bottom. Suddenly my penchant for
theoretical physics regained all its appeal, and I returned to the sofa
determined to read the rest of the book鈥�
I found myself dissatisfied again. The account of the experiment that gives
the book its title does not get to grips with the physics behind toast sliding
off a plate (friction, plus conversion of potential energy into rotational
energy). It leaves the impression that one side of the toast must be heavier
than another to demonstrate Murphy鈥檚 law and land jelly (jam, to Brits like me)
side down. Not so: an unbuttered paperback book demonstrates the principle
perfectly.
For many of the demonstrations, I couldn鈥檛 see much connection with everyday
experience: 鈥淎 ruler balanced horizontally with its mid-point resting on a
cylinder will oscillate with a specific frequency.鈥� So what? Yet opportunities
to make connections with real life went begging: the demonstration that melting
ice cubes don鈥檛 make a tumbler overflow is a great way of showing why the
melting of polar ice floes by global warming makes no difference to sea levels.
In Ehrlich鈥檚 book, it remains a ho-hum demo of Archimedes鈥� principle.
But enough of this: what should you do with your cup of coffee? Here鈥檚
Ehrlich鈥檚 explanation: according to Newton鈥檚 law of cooling, the higher the
temperature of a body is above ambient temperature, the faster it will cool
down. By adding the milk first, you reduce the temperature difference that would
otherwise exist between the coffee and its surroundings, and therefore you slow
its rate of cooling. You also add extra mass, which will tend to slow cooling
still further. Upshot: milky coffee will be hotter when you鈥檝e finished
nattering. Well, it sounds convincing, anyhow.