All those quantities which can be measured directly or indirectly and in terms of which the laws of physics can be expressed are called physical quantities.
For example, length, mass, temperature, speed, force, electric current, etc.
The physical quantities are the building blocks of physics in terms of which the basic laws of physics can be expressed in mathematical forms.
Physical quantities are of two types – fundamental and derived.
(i) Fundamental quantities:
The physical quantities which can be treated as independent of other physical quantities and are not usually defined in terms of other physical quantities are called fundamental quantities.
To give a consistent and unambiguous description to all physical quantities, we need a minimum of seven fundamental or base quantities. These are mass, length, time, electric current, temperature, luminous intensity and amount of substance.
(ii) Derived quantities:
The physical quantities whose defining operations are based on other physical quantities are called derived quantities.
All physical quantities other than the seven base quantities are derived quantities. For example, velocity, acceleration, force, momentum, etc.
Measurement of a physical quantity
The measurement of a physical quantity is the process of comparing this quantity with a standard amount of the physical quantity of the same kind, called its unit.
To express the measurement of a physical quantity, we need to know two things:
(i) The unit in which the quantity is measured.
(ii) The numerical value or the magnitude of the quantity i.e., the number of times that unit is contained in the given physical quantity.
Measure of a physical quantity = Numerical value of the physical quantity × size of the unit
or Q = nu
Let length of a room = 5 m = 500 cm.
Clearly, the smaller the size of the unit, the larger is
the numerical value associated with the physical quantity. Thus, the numerical value (n) is inversely proportional to the size (u) of the unit
nu = constant.
If n1 and n2 are numerical values for a physical quantity Q corresponding to the units u1 and u2, then
Q = n1 u1 = n2 u2