Physics · Mechanical Properties Of Solids · NEET
| Meaning | Internal restoring force per unit area | Fractional change in size or shape |
| Formula | stress = F / A | strain = change / original (e.g. delta L / L) |
| Role | The cause (developed by deforming force) | The result (the deformation produced) |
| SI unit | N/m2 (pascal, Pa) | No unit (pure number) |
| Dimensions | [M L^-1 T^-2] | Dimensionless [M^0 L^0 T^0] |
| Nature | Tensor (can be normal or tangential) | Tensor / ratio, no direction unit |
An outside deforming force causes stress inside the body first. This stress then changes the body's shape or size, and that change is the strain. So the order is: external force -> stress (internal restoring force per area) -> strain (deformation). In NEET wording, stress is treated as the cause and strain as the result. But note: within the elastic limit they rise together and are directly proportional (Hooke's Law: stress = modulus x strain).
Strain = change in length / original length = metre / metre. The units cancel, so strain is a pure number with no unit and no dimension. Stress = force / area = newton / metre^2 = N/m2, which does not cancel, so stress keeps a unit (pascal) and dimensions [M L^-1 T^-2]. This single fact - stress has a unit, strain does not - is the most tested difference in NEET.
Both have the same unit N/m2 and same dimensions [M L^-1 T^-2], which confuses students. Difference: pressure is an EXTERNAL push always perpendicular (normal) to the surface and acts on the outside. Stress is the INTERNAL restoring force per area developed inside the material, and it can be normal (tensile/compressive) OR tangential (shear). Pressure is a scalar; stress is more general (a tensor). So all pressure-like stress is normal, but not all stress is pressure.
Yes. For every kind of stress there is a matching strain. Longitudinal stress -> longitudinal (tensile or compressive) strain = delta L / L. Shearing (tangential) stress -> shear strain = angle theta. Hydraulic (volume) stress -> volume strain = delta V / V. In all three, stress carries the unit N/m2 and strain stays unitless.
Within the elastic region they always come together and are proportional. A special case is thermal stress: if a heated rod is clamped so it cannot expand, the strain is zero (length fixed) but a large compressive stress still develops. This is a favourite NEET trap - fixed length means zero mechanical strain but non-zero stress.
Dimensions of stress are:
A wire is suspended from the ceiling and stretched by a weight W attached at its free end. The longitudinal stress at any point of cross-sectional area A of the wire is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Stress is the internal restoring force per unit area (F/A, unit N/m2), while strain is the fractional change in size or shape (change/original, no unit).
Only stress has units (N/m2 or pascal) and dimensions [M L^-1 T^-2]. Strain is a pure number with no unit and no dimension.
Yes, within the elastic limit. Hooke's Law states stress is directly proportional to strain, and the constant of proportionality is the modulus of elasticity (stress = modulus x strain).
They share the unit N/m2 and dimensions, but pressure is an external normal force per area (a scalar), while stress is the internal restoring force per area (a tensor) and can also be tangential (shear).
SI unit of stress is N/m2 (pascal, Pa) and its dimensional formula is [M L^-1 T^-2] - the same as pressure.
