Q.Distinguish between fundamental units and derived units.

Physical quantities are divided into two categories based on whether their measurement depends on other quantities. A fundamental (or base) quantity is one that does not depend on any other physical quantity for its own measurement — physics recognises exactly seven of these: length, mass, time, temperature, electric current, luminous intensity, and amount of substance. The units used to measure them are correspondingly called fundamental units (metre, kilogram, second, kelvin, ampere, candela, mole in SI).

Every other physical quantity — speed, momentum, force, resistance, pressure, energy, and so on — is a derived quantity: it depends on some combination of the seven fundamental quantities and can always be expressed in terms of them via its own defining formula. The corresponding derived units are built by combining the fundamental units the same way. For instance, since velocity is displacement divided by time, its SI unit is metre divided by second (m/s or m s⁻¹); since momentum is mass times velocity, its SI unit is kilogram-metre per second (kg m/s). …