To maintain consistency in how measurements made around the World are expressed, a system of units called the International System of Units is generally used. This is commonly referred to as the SI system from the French term Système international d'unités.
There are 7 base units in the SI system:
- Length is measured in meters (m)
- Mass is measured in kilograms (kg)
- Time is measured in seconds (s)
- Electric current is measured in ampere (A)
- Temperature is measured in kelvin (K)
- Luminous intensity is measured in candela (cd)
- Amount of substance is measured in moles (mol)
Using these 7 base units all other measurements can be expressed as derived units. For example, velocity which is the distance travelled in a unit time is expressed in m/s, acceleration which is the change in velocity in a unit time is expressed in m/s^2.
The 7 fundamental units can be combined to create a derived unit and they have been found to be sufficient to be used for any measurement that is made.
this answer can look lengthy but its very easy read till thge end and u will find everything regarding ur question in a very easy method...
Fundamental units....i.e also called base units...u can understand it by the following example.
consider a building...the building is made up of bricks of different sizes..these can be considered as the units of the building...which kind of units these may be??? fundamental units off course...the building blocks and the bricks are the minimum components, the combination of which makes the whole building.similarly we can define basic units.DEFINITION: the minimum number of those physical quantities in terms of which other physical quanitites can beobtained is called base quantities. these are decided to be seven in number by the system internationl...i.e
DEIVED QUANTITIES: Now take the same exapmle....this building also composed of some other units that is;
- stores etc etc
look at these units,,,,they are formed by the combination of bricks or building blocks(which was considred as fundamental units in first case).......
these rooms,kitchens, and stores etc can be considered as derived units,,,as its name tells that they can be derived or obtained from other units(from fundamental)...so they are not fundamental but derived from fundamentl.
Q: and how derived are obtained from fundamental?
A: by trhe differnt combination of fundamental....i.e multiplying or dividing two or more than two fundamental...not subtracting or adding...
let us see in examples.
- velocity unit is meter/second...meter and second are fundamental while metr/second is derived.
- newton(derived).... 1 newton = kg m / sec...where kg, m, and sec are fundamental
- coulomb(derived)... 1 coulomb = 1 ampere * sec where both ampere and second are fundamental....
conclusion: different fundamental units joins together (joining in terms of multiplication and division only) and obtain derived units.
Note: there are some units which are neither fundamental nor derived they are called supplemenatry units...i.e radian and steradian..