Derived
units
The
basic units or the fundamental units are independent of
each other. The units of all other physical quantities can
be expressed in terms of these basic units. Such
units are called derived units. Thus, the units,
which are obtained by the combination of fundamental units,
are known as derived units. For example, area can be expressed
in terms of the basic unit of length, as given below:
You know the area of a surface is the product of length
and breadth. Therefore, the unit of area will be equal to
the product of the unit of length and the unit of breadth
(remember that breadth is also length).
Unit of area = metre x metre = (metre)2
Thus, the unit of area is m2.
Similarly, volume is equal to length x breadth x height
of the object.
Therefore, the unit of volume = unit of length x unit of
breadth x unit of height
= metre x metre x metre = (metre)3
Thus, the unit of volume is m3.
The
derived units of other physical quantities are also found
in the same way. Some of the commonly used derived units
are given in Table below.
SI
units and symbols of some derived units
|
Physical
quantity
|
SI
Unit
|
Symbol
|
|
Area
|
Square
metre
|
m2
|
|
Volume
|
cubic
metre
|
m3
|
|
Density
|
kilogram
per cubic meter
|
kg/m3
|
|
Velocity
|
meter
per second
|
m/s
|
|
Acceleration
|
meter
per square second
|
m/s2
|
|
Force
|
kilogram
meter per square second (also called Newton)
|
kg
m/s2 (called N)
|
|
Work
|
kilogram
square meter per square second (also called Joule)
|
kg
m2/s2 (called J)
|
There
are some other commonly used derived units with special
names. they are given in the table below
|
Physical
quantity
|
Special
name
|
Symbol
|
SI
Unit
|
|
Force
|
Newton
|
N
|
kg
m/s2
|
|
Pressure
|
Pascal
|
Pa
|
N/m2
|
|
Energy
|
joule
|
J
|
Nm
|
|
Power
|
watt
|
W
|
J/s
|
