Derived units and quantities

Physical quantities which can be derived from other physical quantities are called derived quantities.

Derived units: The units of physical quantities which can be expressed in terms of fundamental units, for example area, volume etc.


Lets us derive

(i)  unit of force

Force =mass x acceleration.

In SI unit force  (Newton).

In CGS unit force dyne

In MKS unit force is expensed in Newton (N)

In FPS systems force is poundal


(ii)   Energy =mu²

we can substitute the units of m and u to derive unit of energy.

In SI system .

In CGS system ..

1 joule = 10⁷ erg.

In FPS system . poundal.

1British thermal unit (Btu)   =778 ft. Lbf =1054.2 Joule = 252 cal

1kcal = 1000cal.=3.968 Btu

(iii) power (watts or W).



1horse power =550 ft.lbf /s = 746W

1KW =1000W =3414 Btu/hr.

1KW- hr =860 kcal =3414 Btu.