Precision

Those of you that have been watching Marcus du Sautoy's series on Precision, on the BBC, may be interested in a new collection on the elibrary from the National Physical Laboratory.  It provides posters for your lab on the "Standards International" and a useful guide on best practice for dealing with uncertainties.

I've only watched the first episode on length and time so far but it was fascinating to learn how the metre was first defined and the changes in how time and length have been measured over the centuries. 

When encouraging students to take accurate measurements, it's worth mentioning to them that when the Egyptians were building the pyramids, having a cubit of the wrong length was punishable by death.


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Tom Lyons - STE...

Last week, I watched the second programme (on iplayer) on weight.  Marcus was shown measuring the weight of a piece of metal at the bottom of a block of flats, and then measuring again at the top.  The mass appeared to reduce (of course it was the weight changing not the mass) by about 10 milligrams.  This seemed quite a big change to me, but after doing the number crunching, I've worked out that the change from 368.7025 grams to 368.6916 represents a height change of 189 metres - a great demonstration of Newton's law of gravitation.

What did perturb me was that he suggested that "If I took this piece of metal another 100,000 metres up into space then it would weigh hardly anything at all".  Actually, it would change by only 5 grams.  The misconception comes about because astronauts on the International Space Station are "weightless" - but this is because they are in free fall, travelling in orbit around the Earth at a speed of 7.7 km/s!  To weigh less than a gram, you would need to take the chunk of metal to eight times the distance from the Earth to the Moon.