Which of these is NOT one of physics' fundamental quantities?

Weight. Weight is a force, a derived quantity, unlike length, time, and mass, which are fundamental. Read more: Units, Measurement and Dimensions.

Units, Measurement and Dimensions

Strain has no units at all. Divide one length by another, and the units simply cancel out. This article covers dimensions and measurement in physics.

📑 Contents

✊ Must Know

Fundamental Quantities and Dimensions

Seven FundamentalsPhysics has seven fundamental quantities: length, mass, time, electric current, temperature, amount of substance, and luminous intensity. Every other quantity is derived from these.
Scalar vs. VectorA scalar quantity has only magnitude, like speed or mass. A vector quantity has both magnitude and direction, like velocity or force.
Common SI UnitsThe joule measures energy, the pascal measures pressure, the hertz measures frequency, and the newton second measures momentum and impulse.
Astronomical DistancesA light year is the distance light travels in one year. A parsec, about 206,265 astronomical units, is even larger.
Dimensional FormulaA quantity’s dimensional formula expresses it in terms of base quantities: mass (M), length (L), and time (T).

📘 Good to Know

Tiny and Vast Scales, Precision Tools

Tiny UnitsThe angstrom measures atomic-scale distances; the fermi measures nuclear-scale distances. Both are far smaller than a millimetre.
The New KilogramSince 2019, the kilogram has been officially defined using the Planck constant, replacing the old physical metal-cylinder standard kept in France.
Precision InstrumentsA vernier calliper and a screw gauge are both precision tools. A screw gauge typically has a finer least count, useful for very small measurements.

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🌟 Great to Know

Dimensional Analysis in Practice

The Matching RuleTwo quantities can only be added, or set equal, if their dimensional formulas match exactly. This is a powerful, quick way to catch a wrong physics equation.
Dimensionless QuantitiesSome quantities, like strain or relative density, carry no units at all, since they’re built from a ratio of two identical kinds of measurement. Spotting these avoids a common exam trap.
Worked ExampleStrain is a change in length divided by an original length: metres divided by metres, leaving nothing behind. Stress, by contrast, is force divided by area, and keeps real physical units.
Why It Matters PracticallyPrecision instruments like the vernier calliper and screw gauge matter beyond exams too, since measurement error directly affects real experimental results.

📝 Exam Point of View

Exam Point of View

NDA-NA-1 2025, Q139Of stress, strain, pressure, and force, only strain is dimensionless, since it’s a ratio of two lengths (or volumes) of the same kind. View this question on the full NDA-NA-1 2025 paper →
NDA-NA-2 2025, Q53One parsec is about 2 × 10⁵ astronomical units — not 10³, 10⁴, or 10⁶. View this question on the full NDA-NA-2 2025 paper →
NDA-NA-II 2016, Q76A precise-reasoning trap: pressure is scalar specifically because only the force component normal to the surface counts, not simply because “force and area are both vectors.” View this question on the full NDA-NA-II 2016 paper →
NDA-NA-II 2017, Q110A light year measures distance, not time or a quantity of light received. View this question on the full NDA-NA-II 2017 paper →
The PatternThese questions reward precise reasoning over rote recall: knowing WHY a quantity is dimensionless or scalar, not just that it is.

GenSci0010 — Units, Measurement and Scientific Instruments — the general-science overview of SI units and everyday measuring instruments this article’s physics-specific dimensions build on.
GenSci0001 — Physics Fundamentals for Competitive Exams — covers the forces and mechanics whose dimensions this article analyses.
NDA-NA-II 2016 GA Paper — contains the pressure-is-scalar PYQ cited above.
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