NDA Physics · Teaching notes

Units, Measurement and Dimensions — NDA Physics

Units, Measurement and Dimensions is NDA Physics's most reliable scoring chapter — about 14 PYQs across 2017–2025, and roughly three-quarters of them are EASY one-line recall. Almost every question reduces to a fixed fact: a light year is a unit of DISTANCE (asked four separate times), 1 dyne = 10⁻⁵ N, H stands for Henry, 1 kWh = 3.6×10⁶ J, strain is dimensionless. The handful of MODERATE/HARD items just apply one tool — the dimensional formula [M^a L^b T^c] — to find the dimension of G or to identify an unknown quantity (thrust/impulse turns out to be frequency). The chapter teaches in one continuous arc, all inside a single subtopic: first the foundations (physical quantity, unit, the seven SI base units, fundamental vs derived); then the named SI derived units (Newton, Pascal, Joule, Watt, Henry — and the fact that stress and pressure share a unit); the special units of length/distance (light year, ångström, nanometre) and of energy/power (joule, kWh, the force-vs-energy trap); unit-system conversion (CGS ↔ SI, the dyne); and finally the dimensional method itself — writing dimensional formulas, spotting dimensionless quantities, identifying a quantity from its dimensions, and reading least count / precision off an instrument. Memorise the reference tables, learn the one dimensional-analysis recipe, and this chapter is near-free marks.