In order to measure we need to know or define the quantity to be measured and the units for measuring it. In 1971 in Paris France, a system known as the International System of Units (Systeme’ Internationale) with seven basic physical quantities and units were agreed upon as shown in table 1.1 below.
| Basic quantity | SI units | Symbols |
| Length | Meter | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Electric current | Ampere | A |
| Thermodynamic temperature | Kelvin | K |
| Luminous intensity | Candela | Cd |
| Amount of substance | Mole | mol |
before the SI Units were developed, scientists were using different units of measurements depending on their immediate environment.
some of the earlier units of measurements used includes:
- inch which is equivalent to 0.0254 m
- mile which is equal to 1061m
- pint which is equal to 0.57 litres
- gallon which is equal to 4.55 litres
- pound which is equal to 0.45kg
- tonne which is equal to 1000kg
- acre which is equal to 0.41 Hectares
- grams
- centimeters
- seconds
Basic Physical quantities
A physical quantity is a property of a material or system that can be quantified by measurement.
Basic physical quantities are quantities whose property cannot be derived from any other quantity
Other quantities can be obtained from these basic quantities and are referred to as derived quantities.
Derived quantities
Derived quantities are quantities obtained by multiplication or division of basic physical quantities. derived quantities includes:
- Area
- volume
- density
- speed
- Force
- acceleration
- Magnetic flux density
- velocity
- weight
As an example, Area can be obtained from multiplying two lengths together. The rectangle below has its shorter length labelled w and it’s longer length labelled l. it’s area is obtained by multiplying l with w. that is, area= l x w
Area is derived quantity because two lengths are being multiplied together. As an example, consider Speed is defined as distance covered per unit time and is usually given as distance covered/time taken. distance is a length which is a basic quantity and so is the time hence speed is obtained by dividing two basic quantities together.
Related Topics
- Measuring Length
- Using meter rule
- Area
- Area of irregular surfaces
- Volume
- Volume of irregular solids
- Measuring mass
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