The meaning of Pi/ Plastic bags/ Inertia/ Slugs
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Q Is the calculation of pi empirical (by measurement) or theoretical? (asked by Rev John Wilcox)

Pi is a constant - but it is an "irrational" number, meaning it cannot be exactly expressed as the ratio of two numbers. So it can never be completely enumerated, only approximated: it is the sum of an infinite series. But the fact that pi is constant has been known for so long that it is quite untraceable. In very ancient times, 3 was used as the approximate value of pi - almost certainly derived from measurement.

It seems that Archimedes in the 3rd century BC was the first person to make a scientific effort to compute it. By calculating the circumference of a 96-sided polygon, he showed the value was between 223/71 and 22/7 - that is, to 1 per cent accuracy.

The precision of pi has increased steadily throughout history but it wasn't until the introduction of computers this century that pi could be calculated to many decimal places. Nowadays computer algorithms can express it to millions of digits.

Q How are plastic bags made?

All plastic bags, sacks and bigger are made by a process know as "blown film extrusion", in which the molten plastic is blown up like a balloon as it is stretched out to produce a continuous tube of the film. This film is flattened to make a continuous double layer - which is what makes new bags difficult to open sometimes. This is then printed, and cut to the appropriate length, sealed and a handle is cut out, all in one process which is continuously repeated to produce individual bags.

Q If all the matter in the universe was concentrated into such a small space just after the Big Bang, why didn't gravity just pull it all back together again?

Because of inertia. The primieval stuff was flying apart so fast after the inflationary push that gravity could not pull it back instantaneously. If the universe were "closed" then eventually, after all that outward momentum had been used up, gravity would triumph and the universe would close up again. But recent measurements suggest the universe contains only 20 per cent of the matter required to pull it together again, so it will continue expanding forever.

Q How long do slugs live?

The life expectancy of slugs varies greatly, depending on type, size and so on. But in general, very small slugs live about six months, while very large slugs can live for between eight and 10 years.

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