Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.We will deal with carbon dating first and then with the other dating methods.
By measuring the ratio of the radio isotope to non-radioactive carbon, the amount of carbon-14 decay can be worked out, thereby giving an age for the specimen in question.
But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.
The technique hinges on carbon-14, a radioactive isotope of the element that, unlike other more stable forms of carbon, decays away at a steady rate.
Organisms capture a certain amount of carbon-14 from the atmosphere when they are alive.
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We note that at the instant the swimmer touches the edge of the pool our wristwatch reads and 53 seconds.
How long has the competitor taken to swim the 1,500 metre race?
Without the starting time it is Actually, knowing the starting time is still not enough.
During the race you have to watch the swimmer and count how many laps he has swum so you know that he has done 1,500 metres.
A swimming race illustrates the simple principles involved in measuring time.