From P. MOSTYN
I was dismayed to find basic misconceptions reinforced in ‘Making the
most of weightlessness’ (11 July).
The statement that ‘the influence of gravity on the orbiting craft will
be one hundred-thousandth (10 -5) of what it is on Earth’ at
an altitude of 515 kilometres is quite wrong. A simple calculation shows
that it will be 85.6 per cent of its value at the surface of the Earth.
Had the article left it at that I might have put it down to a rather dubious
oversimplification to save space, but the ‘explanatory’ box on the next
page compounds the error at length.
Freefall is not the same as weightlessness, although for many purposes
the effects are the same. To be truly weightless you must either have zero
mass or be in a gravitational field of zero strength, neither of which conditions
holds in a drop tower. Once your speed has increased to the point where
air resistance is equal to your weight you have reached your terminal velocity
and might as well be standing on the ground (until you hit it).
The worst error is the statement that ‘. . . the centrifugal force on
the craft counters the gravitational force . . .’ which is so wrong that
it is hard to know where to start. If the statement were true, there would
be no force on the craft, therefore nothing to keep it in orbit and it would
float aimlessly off into space. The correct explanation is that the only
force on the craft is its weight, caused by the gravitational field of the
Earth. This unbalanced force provides the centripetal force needed to cause
orbital motion. One final point. It is much more helpful to use units of
newtons per kilogram than metres per second per second for g as this keeps
the physics of the situation more clearly in mind.
Advertisement
The school where I teach science will continue to subscribe to your
invaluable magazine, but I will gleefully use the article in question as
a critical comprehension, and an example of the non-infallibility of the
printed word.
P. Mostyn Hemel Hempstead, Herts
