Physical quantity
What is physical quantity ? give examples !
A quantity which can be measured directly or indirectly is called a physical quantity. Example :- mass of an object, Length of an object, Time of an event, Velocity, Acceleration, Force etc.
Classification of physical quantity
Physical quantity can be classified into mainly two types.
- Fundamental quantity
- Derived quantity
Fundamental quantity
What is fundamental quantity ?
Those physical quantity which are independent of each other are known as fundamental quantity . These fundamental quantities do not take support from other physical quantity for its measurement .
It is also known as a basic quantity. there are seven fundamental quantity.
- Mass
- length
- Time
- Temperature
- Electric current
- Luminous intensity
- Quantity of matter
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Derived quantity
What is derived physical quantity ?
Any physical quantity which can be derived from the fundamental quantity by multiplying or dividing them is called derived physical quantity.
Example :- speed , force etc.
Basic concept
Mass
Mass of a body is defined as the quantity of matter or material content in a body.
The mass of a body is constant and does not change from place to place . Mass of a body cannot be zero. S I unit of mass is kg.
Weight
The weight of a body is the force with which the body gets attracted toward the centre of the earth by the action of gravitational pull.
SI unit of weight is Newton.
Difference between mass and weight
| Mass | Weight |
|---|---|
| Total matter contained in a body. | It is a force by which a body is attracted towards the centre of earth. |
| It is a scalar quantity. | It is a vector quantity. |
| It is constant for a body at all places. | It is change from place to place ( one planet to another planet ) |
| It is never zero. | It is zero at centre of earth. |
| SI unit - kg | SI unit - Newton |
Length
The distance between two points in space is called length SI unit of length is Meter.
Time
Time is a measure of the duration between the occurrence of two events . SI unit is second.
Unit
To measure a physical quantity and to express its measure we require a standard of that physical quantity. This standard is called unit of that physical quantity.
For the measurement of a physical quantity ( say x ) two things are required :
1. Numerical value ( n )
2. The unit ( in which the physical quantity is expressed ) { u }
i.e. Physical quantity = Number × Unit
X= nu
- Smaller the unit larger will be the numerical value.
Characteristics of unit
- It should be well defined
- It should be acceptable internationally. ( It should not change from place to place or time to time. )
- It should be of suitable size.
- It should not change with the physical conditions such as temperature , pressure etc.
Types of unit :
There are three types of unit .
- Fundamental unit
- Supplementary unit
- Derived unit
Fundamental unit
A fundamental or basic unit is that which is independent of any other unit . The unit in which fundamental quantities are expressed are known as fundamental unit.
There are seven fundamental unit .
| s.no. |
Unit | physical quantity |
|---|---|---|
| 1. | Kilogram | Mass |
| 2. | Metre | Length |
| 3. | Second | Time |
| 4. | Kelvin | Temperature |
| 5. | Ampere | Electric current |
| 6. | Mole | Quantity of matter |
| 7. | Candela | Luminous intensity |
Supplementary unit
It is referred as a dimensionless unit which is employed with the fundamental units to create the derived units.
| s.no. | physical quantity | name of unit |
|---|---|---|
| 1. | Phase angle | Radiun |
| 2. | Solid angle | Steradian |
Derived unit
Derived units are those which depend on the fundamental unit. or which can be expressed in the terms of fundamental unit .
The unit that consists of the combination of the fundamental units are called derived units.