Units and Measurements

Syllabus Unit: Mechanics

Why it matters

Chapter overview

Every physics quantity NEET asks about is built from a handful of base units, and every measurement carries some uncertainty — this topic covers the SI system and dimensional analysis that let you check and derive formulas, the error/significant-figure rules that let you report a measurement honestly, and the vernier callipers and screw gauge that push measurement precision below what a plain ruler can achieve.

Physics Syllabus Mechanics 30%–35% weightage

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SI Base Units

1Length (m)Fundamental unit of distance

2Mass (kg)Fundamental unit of matter

3Time (s)Fundamental unit of duration

4Current (A)Fundamental unit of electric current

1SI Units & Dimensional Analysis

Every physical quantity is expressed as a base unit raised to a power — the SI system's seven base units are the foundation NEET expects you to know without hesitation.

Learning Objectives

List the seven SI base units and their symbols (m, kg, s, A, K, mol, cd).
Write the dimensional formula of a derived quantity from its defining equation.
Apply the principle of homogeneity to check whether a physical equation is dimensionally consistent.
Distinguish fundamental (base) quantities from derived quantities.

Key Terms

SI SystemThe International System of Units, built on seven base units: metre, kilogram, second, ampere, kelvin, mole, candela.

DimensionThe power to which a base quantity is raised in expressing a derived quantity, e.g. [M][L][T]⁻² for force.

Dimensional FormulaAn expression showing the powers of M, L, T (and other base dimensions) that define a physical quantity.

Principle of HomogeneityStates that only quantities with the same dimensions can be added, subtracted, or equated in a valid physical equation.

Physics needs a common, internationally agreed set of units so that a measurement made in one lab means the same thing everywhere. The **SI system** (Système International) fixes seven such base units, from which every other unit in mechanics, electricity and thermodynamics is derived.

The seven SI base units

Metre (m) — length
Kilogram (kg) — mass
Second (s) — time
Ampere (A) — electric current
Kelvin (K) — thermodynamic temperature
Mole (mol) — amount of substance
Candela (cd) — luminous intensity

Writing dimensional formulae

Every derived quantity can be written as a formula involving mass [M], length [L] and time [T] (plus other base dimensions where relevant). For example, velocity = displacement/time gives dimensions [L][T]⁻¹.

[F] = [M][L][T]⁻² (from F = ma)

Exam TipNEET often asks for the dimensional formula of less common quantities — Planck's constant, gravitational constant G, surface tension, coefficient of viscosity. Memorize the standard set.

Principle of homogeneity & its uses

Checking the correctness of a physical equation
Deriving a relationship between quantities (dimensional analysis)
Converting a physical quantity's value between systems of units

Common MistakeDimensional analysis can never determine a dimensionless numerical constant (like the ½ in s = ut + ½at²) — a common NEET trap.