Sunday, March 27, 2016

Kids’ Gravitational Physics

‘I read very, very little fiction as a kid. All the books I can remember are junior science books.’

These are the words of English writer Mark Haddon (1962-), whose award-winning 2003 novel The Curious Incident of the Dog in the Night-Time captivated my younger son, both in print and in a recent stage production. It is a story to which my son can readily relate, for he and the novel’s young protagonist share a defining trait: Asperger’s syndrome. My son even bought a commemorative Curious Incident pencil case, bearing the phrase: ‘I like Maths and being on my own.’

For this reason alone, and writing from a scientist’s angle, it gives me tremendous pleasure to see that science museums the world over are now laying great emphasis on interactive exhibits designed to captivate ‘Aspie’ youngsters, who see their world in extraordinary detail, identify patterns where their peers see only randomness, and generally analyse everything to the nth degree.

There are around fifty such science museums in Britain alone. Alongside these are play facilities aimed primarily at children, such as The Puzzling Place, in Keswick and Techniquest (Figure 93.1), based in Cardiff. I took Junior to the latter this Easter weekend. The interactive exhibits, demonstrating sound, optics, anatomy and magnetism, kept us occupied for hours. One particular piece practically mesmerized him. It was, at first view, a plastic bowl with two identical holes at its deepest points (Figure 93.2). Kids of all ages took turns dropping plastic balls into the bowl, watching carefully as each one swirled round and round, often circling one hole before switching, at the last moment, to the other. To the unscientific eye, the eventual destination of each ball was random. It was simply fun to watch a moving object with (apparently) a will of its own.

Figure 93.1: Further details are available at:

Copyright © 2016 Techniquest

Figure 93.2: Watching the ‘magic’ of gravity

Copyright © 2016 Paul Spradbery

Junior, to his credit, saw things his own way. After studying the sweeping movements of ball after ball, he remarked: ‘Dad, why is it, when they slow down a bit, they sometimes start going round like a number eight?’

It was an astute observation. Of all the questions he might have asked, though, this was perhaps the most difficult for me to answer. Still, as Einstein once said: ‘If you can’t explain it to an eight-year-old, then you don’t understand it yourself.’

So here goes. Whenever a ball rolled deep into one of the holes, it would move along a circular path, round and round, in a downward spiral, until it disappeared from view. However, if a ball were moving with sufficient speed at a high enough point, then it might have the energy required to roll across to the other hole. Then, if it had lost too much energy by so doing, it would stay there and eventually drop through that hole. If sufficient energy remained, it might be able to switch back again, and so forth until its energy finally ran out. This two-hole arrangement is called a ‘binary system’ and plays a crucial role in gravitational physics.

Why a figure-of-eight? If a ball were to ‘escape’ from one orbit in, say, a clockwise movement, it would necessarily join the other orbit in an anticlockwise movement (Figure 93.3).

Figure 93.3: This simple graphic shows a figure-of-eight orbital pathway operating within a binary system.

Copyright © 2005 Pearson Prentice Hall, Inc.

The so-called Roche lobe is a region around a star, within a binary system, where the orbiting object is gravitationally attracted to that star. The lobe is shaped like a teardrop, and, when conjoined to its corresponding lobe, forms a figure-of-eight. The orbit takes its path along a line of equal gravitational potential, which is analogous to a geographical contour line or a meteorological isobar. The crossover point from one Roche lobe to the other is called the L1 Lagrangian point of the orbital system.

Perhaps – just perhaps – many years from now, there will be a banknote bearing the image of a world-renowned scientist whose inspiration stemmed from playing with a toy of such supreme scientific elegance. 

Copyright © 2016 Paul Spradbery

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