Types of units | Fundamental and derived units | unit and measurements-2

Types of units | Fundamental and derived units | units and measurements-2

Types of unit:

Units are derived in two ways -

• Fundamental units

• Derived units

Fundamental units:

                                

Those units which are self derived and can neither be resolved into any other units, known as fundamental units.

        Fundamental units are used to measure fundamental Physical quantities ( Fundamental physical quantities are those qualities which are self defined. Ex. Mass, length, time etc ). If we  want to measure mass, length or time, then they are independent units and measuring these units do nott require any other units.

There are 7 fundamental units of, in the table below 7 fundamental physical quantities along with their units are mentioned.

7 Fundamental physical quantities

Derived units:

The units of various physical quantities can be expressed in terms of fundamental units. These units are known as derived units.

For example, if you want to measure the unit of velocity then it depends upon the distance and time. So we define the unit of velocity in terms of units of length and time, i.e. ms-1. These physical quantities which require fundamental quantities to derive themselves, are known as derived physical quantities.

Examples : 

Physical quantity                    Unit

     velocity                                 ms-1

  acceleration                             ms-2

      force.                                 kg ms-2

  momentum                            kg ms-1

   

And so on…

We can conclude from other examples that units of these derived quantities are expressed in terms of fundamental units.

Unit of length:

The length of an object is defined as the distance between the two ends of the object.

Its SI Unit is metre ( symbol:-  m ).

The internationally accepted definition for metre is : the distance traveled by light in a vacuum in 1/299,792,458 second.

Unit of mass:

Mass of a body is the quantity of matter inside  it.

SI Unit of mass is Kilogram ( kg )

Mass is an essential property for a body, so it can never be zero. Further it is always constant and unaffected by pressure, temperature or place. 

Following definitions of mass were internationally accepted:

• 1 kg was considered as the mass of one cubic decimetre of water at 4°C ( at 4°C the density of water is maximum ).

• According to general conference of weights and measures kilogram is defined as the mass of a Platinum iridium cylinder kept at the International Bureau of weights and measures in France.

Unit of time:

According to Einstein, time is simply what a clock reads.

SI unit of time is second ( s ).

According to Paris Academy of science, second is defined as the time taken by a simple pendulum of 1 metre length to swing from one extreme position to another.

There is also another definition for time but due to variation in values gravity and other quantities, these definitions are not perfect.

So we consider the simple definition of time given by Einstein.