Key Concepts: Physical Quantities & Units

Physical Quantities

A physical quantity is any property of a material or system that can be measured. Every measurement has two parts: a numerical value and a unit.

• Base Quantities: The seven fundamental quantities defined by the SI system — length (m), mass (kg), time (s), electric current (A), temperature (K), amount of substance (mol), and luminous intensity (cd).
• Derived Quantities: Quantities formed by combining base quantities through multiplication or division — e.g., speed (m/s), force (kg·m/s² = N), energy (kg·m²/s² = J).

Why Units Matter

• Units ensure measurements are meaningful and comparable. A number without a unit (e.g., "5") has no physical meaning.
• Consistent units prevent errors in calculations — always convert to SI before computing.
• Unit analysis can help verify that an equation is dimensionally correct.