Physical Quantities

A physical quantity is a quantity which can be measured and consists of a magnitude and unit. For example length, weight, volume etc are physical quantity. By using these physical quantities we can find some basic attributes of an object. Physical quantities have at least two things in common. One is the size or magnitude and the other is the unit in which the quantity is measured. For example, to describe a brick, its length, width, height and mass are measured.
Physical quantities are classified into two types:
  • Base quantities
  • Derived quantities

Base Quantities

Those quantities which do not depend on other quantities are the base quantities. Or in other words base quantities are the quantities on the basis of which other quantities are expressed. There are seven Base quantities. These are length, Mass, Time, Electric Current, Temperature, Luminous Intensity and Amount of Matter.
Fig. 1: Fundamental Quantities with units and symbols.

Derived Quantities

Those quantities which are dependent or expressed in terms of base quantities are termed as derived quantities. Examples of derived quantities are Area, Volume, Speed, Force, Work etc.
Fig. 2: Derived Quantities with units and symbols.
A derived quantity has an equation which links to other quantities.
It enables us to express a derived unit in terms of base-unit equivalent.
$$F = ma \, \text{; Newton} = kg \, m \, s^{-2}$$
$$P = F/A \, \text{; Pascal} = kg \, m \, s^{-2}/ m^2 = kg \, m^{-1} \, s^{-2}$$

Scalar Quantities

A physical quantity which can be expressed only by magnitude (By a numerical value) is known as scalar quantity. Mass, Time, Temperature, Speed, work are scalar quantities.

Vector Quantities

Those quantities which need both magnitude and direction to express the quantity completely and also follow the laws of vector algebra are the Vectors.
Fig. 3: Scalar Quantities
Fig. 4: Difference between scalar and vector

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