Physical Quantities & SI Units — A/L Physics

Physical Quantity A property of a material or system that can be measured and expressed numerically. Every physical quantity consists of a numerical value and a unit. For example, a length of 5.0 m means the quantity is 5 times the standard metre. Without a unit, the number is meaningless in physics.

Base Quantity A fundamental physical quantity that cannot be defined in terms of other quantities. The SI system defines seven base quantities: length (m), mass (kg), time (s), electric current (A), thermodynamic temperature (K), amount of substance (mol), and luminous intensity (cd). All other quantities are derived from these. For example, speed is derived from length and time.

Derived Quantity A physical quantity expressed as a combination of base quantities using multiplication or division. Derived quantities have derived SI units. For instance, velocity = displacement/time, so its SI unit is m s⁻¹. Force = mass × acceleration, giving the newton (N = kg m s⁻²). Always check the base units of a derived quantity to verify equations.