DERIVED UNITS

DERIVED UNITS

Definition:

SI units of all other physical quantities derived from the base and supplementary units are called derived units.

For example, the unit of velocity, acceleration, force, work and momentum etc. are the derived units because the units of these quantities are the combination of two or more base units. Some of the derived units are given in the following table:

Physical Quantity	Unit	Symbol	In term of base units
Force	Newton	N	Kg ms-1
Work	joule	J	Nm = Kgm2 s-2
Power	watt	W	Js-1 = Kg m2 s-3
Pressure	Pascal	Pa	Nm-2 = Kg m-1 s-2
Charge	coulomb	C	As

Some common Examples of Derived Units

i) Speed

It is defined as the distance covered in unit time. In SI units the unit of distance is meter (m) and unit of time is second (s) therefore

Speed =  = m/s = ms^-1 Hence SI unit of speed is ms^-1

ii) Acceleration

It is defined as the change in velocity per unit time. It is denoted by “”

Acceleration = = m/s²= ms^-2

iii) Force

It is defined multiplication of mass and acceleration. It is denoted by “F”

Force = mass x acceleration = a m = m x  = kg x m/s² =  Kgms^-2

iv) Work

It is defined the distance traveled by the body under the action of force.

Work = force x distance = N-m i.e. joule

v) Momentum

It is defined the quantity of motion in a body.

Momentum = mass x velocity = Kg x m/s = Kgms^-1