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Weight - was originally defined in Britain in terms of ounces & pounds. The origin of these terms is not very clear. Other cultures must have had their own units.
Later the unit of weight was defined in terms of the weight of a unit volume of pure water. Hence the weight of 1 Cubic Cm (which was also called Milli Litre) was defined as 1 Gram. Water was taken as the reference because of its universal availability.
The interconnection between volume & weight units can easily be remembered as shown in the following example.
A litre can be imagined as the volume of a cube whose sides are 10 cm each. Each cc of water weighs 1gm. There are 1000 cc's in a litre. Hence a litre of water weighs 1 Kg.
Hence 1cc is also called a Milli Litre (ml) (1/1000th of a litre).
1 cubic meter has 1000 litres. 1 cubic metre of water weighs 1000 kgs or 1 Tonne.
Angle
The complete angle was related to the movement of the Earth around the Sun in a year. Possibly the complete angle was seen as made of 360 degrees since the year was calculated as consisting of 360 days.
Angles are also measured in radians. In radians, a complete angle is 2π and the sum of the angles of a triangle is π.
The Very large & the Very Small
With the advancement of nuclear science & astronomy and digital technology, there has been a need to develop units which can be used to measure very large & very small distances and time periods. The International System has also been extended to accommodate these needs.
These units have been described in an earlier chapter 17.9 on "Decimals in Daily Life".
Derived Units
Derived units are built from the basic units and they usually reflect the "rate" between 2 different kinds of units.
Speed = Distance/ Time - Rate at which the distance changes w.r.t time
Density = Weight / Volume - Rate at which the wright changes w.r.t volume
Non-Liner Scales
Some of the physical phenomena cover such a huge magnitude that a linear scale would be to impractical to measure & report. For these non-linear scales are used.
One such example is the measurement of the magnitude of earthquakes for which a Richter Scale is used. It was developed by Charles Francis Richter in collaboration with Beno Gutenberg around 1935.
A reading of 2 is of the order of 10^2 (100) whereas a reading of 3 is of the order of 10^3 (1000). Hence a reading of 8 (10^8) is 1000,000 (10^6) more powerful than a reading of 2!
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