International System of Units (SI) for basic and derived quantities.
Products such as sugar, milk, salt are purchased from shops in specific quantities or units of measure.
Basic and derived units of measurement.
In 1971 the fourth General Conference on Weights and measures identified seven (7) quantities as basic quantities. This formed the basis of the International System of Units abbreviated as SI units from French name Syteme Internationale d’unites.
What are Basic quantities?
A basic quantity is one whose unit can be defined without referring to other quantities.
The basic quantities are:
Length.
Mass.
Time.
Temperature.
Electric current.
The table below shows the basic quantity unites and their symbols.
Basic quantity | SI unit | Symbol |
Mass | Kilogram | kg |
Length | Metre | m |
Time | Second | s |
Temperature | Kelvin | K |
Electric current | Ampere | A |
What are Derived Quantities?
Derived quantities are quantities that are calculated from two or more measurements.
Derived quantities cannot be measured directly.
Derived quantities an only be calculated. Examples of derived quantities are:
Volume.
Density.
Area.
Volume =length x width x height (m x m x m) or m3 (cubic metres).
Density = Mass / Volume ((Kg/m3)
Area (square or rectangle) = Length x width (m x m) or m2 (square metres)
Applying the International System of Units (SI)
Mass.
Mass is the amount of matter in an object.
Mass is measured in milligrams (mg), grams (g), kilograms (kg) and tonnes (t).
Length.
Length is defined when we measure how long something is.
It is the distance between two points.
The units for measurement of length are millimeter (mm), centimeter (cm), Metre (m) and Kilometres (km)
Time.
Time is measured using a digital or analogue clock.
The basic unit of measuring time is seconds (s).
Larger units of time are minutes (min) and hours (hr).
Temperature.
An instrument that measures temperature is called a thermometer.
Temperatures is measured in
Degrees Celsius (0C.)
Degree Fahrenheit (0F.)
Kelvin (K)
The thermometer that measures our body temperature id called a clinical thermometer. The scale used or unit of measurement used is the Celsius scale (0C.)
A clinical thermometer reads temperatures from 35oC to 42 oC
There are other thermometers that measure temperature of other materials such as laboratory thermometer which has a range from -10 oC to 110 oC.
Electric current.
An electric current is the rate of flow of charge through a given point in an electric circuit.
The SI unit for measuring the magnitude of an electric current is Amperes (A).
Area.
Area is the measure of the size of a surface.
Two length measurements usually describe area hence presented in square units.
The basic unit of measuring area is square metres (m2)
Area can also be expressed in square millimeters (mm2), square centimeters (cm2) and square kilometres (km2).
Area of regular shapes
Types of formula | Shapes |
Triangle
Area =1/2 x base x perpendicular height =1/2 bh | |
Rectangle Area=length x width =lw | |
Trapezium
Area = Area =1/2 (sum of parallel side) x perpendicular distance between the lines =1/2 (a+b)h | |
Circle Area = 𝜋𝑟2 |
Area of irregular shapes.
Area of an irregular shape can be estimate by first sub dividing the shape into small regular shapes.
The small regular shapes used can be of 1 cm length.
The area of the whole shape can be calculated as follows.
Count the number of whole squares.
Count the squares which are either half or more than half.
Add the total number of complete squares to half the total number of the incomplete squares. For example, in the above picture
Full squares=5
Number of squares that are more than half or half =6 then divide by 2 =3 Approximated are=5+3 = 8 square units.
Volume.
It is the amount of space an object occupies.
Objects can be solids like a brick or liquid like water.
Apparatus that measure volume of a liquid include;
Measuring spoon.
Measuring jug.
Measuring cylinder.
The SI unit for measuring volume is cubic metre (m3)
Volume can also be measured in other units such as;
Cubic centimetres (cm3)
Cubic millimetres (mm3)
Common units for measuring liquid volume include
Litres, Millilitres.
Density.
It is the heaviness of a substance in relation to their volume. Therefore, density is mass per unit volume.
Mass can be measured in kilograms (kg) while volume is measured in cubic metre (m3)
Since mass is measured in kg and volume is measured in m3, the SI unit for density is
kg/m3.
Density can also be expressed in grams per cubic centimetre (g/cm3)
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