Four categories of physical quantities

Based on dimensions, physical quantity can be classified under four categories. Dimensional constants E.g., gravitational constant (G), Planck's constant (h) and velocity of light (c) Their values are always constant.

Gravitational constant

The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal gravitational constant, Newton's constant, and colloquially Big G. It should not be confused with "little g" (g), which is the local gravitational field (equivalent to the free-fall acceleration), especially that at the Earth's surface; see Earth's gravity and Standard gravity. Non-dimensional constants The constants having no dimensions. E.g., p, e, 1, 2,3 ..............etc.

Dimensionless variables

Angle, specific gravity, strain etc., do not have any dimensions and any constant value and hence, are variables.

• Angle

In geometry, an angle (in full, plane angle) is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. The magnitude of the angle is the "amount of rotation" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide with the other . Where there is no possibility of confusion, the term "angle" is used interchangeably for both the geometric configuration itself and for its angular magnitude (which is simply a numerical quantity).

• specific gravity

Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water. The term "relative density" is often preferred in modern scientific usage.

Dimensional variable

Velocity, acceleration, density, area, volume, force, etc., have no constant value, but have dimensions.