Physics · Mechanical Properties Of Solids · NEET
Strain is a ratio of two quantities that have the SAME unit. Longitudinal strain = change in length / original length = ΔL / L. If ΔL is measured in metres and L is measured in metres, then metre ÷ metre = 1. The units cancel. The same happens for volume strain (m³ ÷ m³) and shear strain (angle, which is also a pure number). So strain is always a pure number with no unit attached.
ΔL is just the actual stretch, measured in metres (it HAS a unit). Strain is ΔL divided by the original length L, so it is a fraction (no unit). Example: a 2 m wire stretched by 0.002 m has ΔL = 0.002 m but strain = 0.002 / 2 = 0.001. NEET options often mix these up — read carefully whether the question asks for the elongation or the strain.
Both. Since it is length ÷ length, its dimensional formula is [M⁰ L⁰ T⁰], which means zero powers of mass, length and time — that is dimensionless. And because there is no unit left after the metres cancel, it is also unitless. Any quantity made by dividing two same-type quantities is both unitless and dimensionless (like refractive index, Poisson's ratio, or relative density).
(1) Longitudinal strain = ΔL / L (change in length / original length). (2) Volume strain = ΔV / V (change in volume / original volume). (3) Shear strain = the angle of tilt θ (approximately Δx / L, sideways shift / height). All three are ratios of like quantities, so all three are dimensionless.
No. Strain is a pure number (a ratio), so it is a scalar, not a vector. It only tells you HOW MUCH the shape changed relative to its original size, not a direction. Note the contrast: stress is more complex (a tensor), but strain, the way NEET uses it here, is just a dimensionless number.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Strain is how much a body changes shape or size compared to its original size. It is found by dividing the change (like the stretch ΔL) by the original value (like the original length L).
Strain has no SI unit. It is a ratio of two lengths (or two volumes), so the units cancel and it becomes a pure number.
The dimensional formula of strain is [M⁰ L⁰ T⁰], meaning it is dimensionless. All powers of mass, length and time are zero.
For NEET at this level, strain is treated as a scalar — a pure number with no direction. It only gives the amount of deformation relative to the original size.
Strain appears in the definitions of Young's modulus, bulk modulus and shear modulus (each is stress ÷ strain). Knowing strain is dimensionless helps you check formulas and quickly reject wrong options that give strain a unit.
