Numbers - where do they come from?  (Page 4/5)

5.2 On a typical lightweight bed sheet, there might be about three threads per millimetre, both across and lengthwise. If a sheet for a double bed measured two metres square, that would mean 6,0 × 10 ³ threads across plus another 6,0 × 10 ³ threads lengthwise. That gives us 1,2 × 10 ⁴ threads, each about two metres long. Calculate how many kilometres of thread it took to make the sheet. Tonight, measure your pillowslip and do the same calculation for it.

5.3 A typical raindrop might contain about 1 × 10 ^–5 litres of water. In parts of South Africa the annual rainfall is about 1 metre. On one hectare that means about 1 × 10 ¹² raindrops per year. On a largish city that could mean about 6 × 10 ¹⁶ raindrops per year, or about 1 × 10 ⁷ drops for every man, woman and child on Earth. How many litres each is that?

5.4 Calculate: (give answers in scientific notation)

5.4.1 $\frac{\mathrm{3,}\text{501}\times {\text{10}}^{-5}}{\mathrm{9,5}\times {\text{10}}^{-8}}+\mathrm{4,3}\times {\text{10}}^{-\text{11}}$

5.4.2 $\frac{\mathrm{3,5}\times {\text{10}}^6+\mathrm{1,4}\times {\text{10}}^{-\text{17}}}{\mathrm{3,5}\times {\text{10}}^6-\mathrm{1,4}\times {\text{10}}^{-\text{17}}}$

end of CLASS WORK

We use prefixes, mostly from Latin and Greek, to make names for units of measurement. For example, the standard unit of length is the metre . When we want to speak of ten metres, we can say one decametre; one hundred metres is a hectometre and, of course, one thousand metres is a kilometre. One tenth of a metre is a decimetre; one hundredth of a metre is a centimetre and one thousandth is a millimetre. There are other prefixes – see how many you can track down.

Your computer pals will be able to confirm, I hope, that in computers a “kilobyte” is really 1024 “bytes”. Now, why is it 1024 bytes and not 1000 bytes? The answer lies in the fact that computers work in the binary system and not in the decimal system like people. Try to find the answer yourself.

Assessment