Anything that can be numerically measured using an instrument is a physical quantity. E.g. length, time, speed, density etc.
Physical Quantities can be of two types:
Fundamental: Physical quantities which cannot be explained by other physical quantities are called fundamental physical quantities. E.g. length, time, mass, temperature etc.
Derived: Physical quantities which are explained on the basis of fundamental physical quantities are called derived physical quantities. E.g. volume, speed, density etc.
Units: Units are used to describe physical quantities mathematically.
International System of Units (SI Units): The SI units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities. It is also knows as M.K.S system.
