Physics · Mechanical Properties Of Solids · NEET
By definition in NCERT, stress is the internal restoring force per unit area that the body develops to oppose the deformation. When the body is in equilibrium, this restoring force is equal in magnitude to the applied deforming force. So numerically stress = F / A, where F is the applied force, but conceptually it measures the internal force the material sets up per unit area to resist the change in shape or size.
Because the same force affects a thin wire and a thick wire very differently. A 10 N load on a thin wire may snap it, but the same 10 N on a thick rod does almost nothing. Dividing by area (stress = F / A) gives a quantity that describes the material's internal condition, not just the load. This is why stress, not force, decides whether a material breaks.
Stress is not a simple vector. It is a tensor, because its value depends on both the direction of the force and the orientation of the surface it acts on. For NEET numericals you usually treat the magnitude as stress = F / A, but remember NCERT calls it a tensor, not a vector. This is a common one-line MCQ trap.
Both have the same formula (force per unit area) and the same SI unit (N/m or pascal). The difference: pressure always acts perpendicular and pushes inward (compressive) on a fluid, while stress in a solid can be tensile (stretching), compressive (squeezing) or shearing (tangential). Pressure is one special case; stress is the more general idea.
Less stress. Since stress = F / A, if the force F is fixed and you increase the area A, the stress goes down. That is why cranes and bridges use thick cables — a larger cross-section lowers the stress so the material stays within its safe elastic limit.
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.
The SI unit of stress is newton per square metre (N/m), which is given the name pascal (Pa). Because real materials have huge stress values, you often see it in megapascal (MPa = 10^6 Pa) or the equivalent N/m.
Stress = restoring force / cross-sectional area, written as stress = F / A. Here F is the force acting on the surface and A is the area of that surface, measured perpendicular to F for longitudinal stress.
The three types are tensile/compressive (longitudinal) stress from a force perpendicular to the surface, shearing (tangential) stress from a force parallel to the surface, and hydraulic (volume) stress from pressure acting from all sides.
Yes. Both stress and pressure have the dimensional formula [M L^-1 T^-2], because both are force per unit area. This is why they share the same SI unit, the pascal.
No. Although it involves a direction, stress is a tensor, not a vector, because its value depends on both the direction of the force and the orientation of the surface. Treat its magnitude as F / A in numericals but remember it is technically a tensor.
