Mechanical Properties of SolidsNEET Physics · Class 11 · NCERT Chapter 8

You can expect 1 to 2 questions from this chapter in NEET 2027. The chapter has medium PYQ frequency. Young's modulus calculations, the stress-strain curve, bulk modulus and elastic PE in a wire are the most heavily tested concepts.

Stress is the restoring force per unit area inside the body, with units of pascal (N per m squared). Strain is the fractional deformation, dimensionless. Stress is the cause; strain is the response. Hooke's law says they are proportional within the elastic region: stress equals modulus times strain.

Young's modulus Y equals longitudinal stress divided by longitudinal strain. It measures resistance to length change under axial load. Steel has Y about 2 times 10 to the 11 pascals; rubber has Y about 1 times 10 to the 7 pascals. Higher Y means stiffer material — more force needed for the same fractional stretch.

Bulk modulus B equals volumetric stress (pressure) divided by volumetric strain (fractional volume change). It measures resistance to compression from all sides — relevant for fluids and solids under hydrostatic pressure. Young's modulus describes change in length under axial load. They are different quantities of the same material.

When you stretch a wire, it gets longer along the pull and slightly thinner across. Poisson's ratio sigma equals minus the lateral strain divided by the longitudinal strain. For most materials, sigma is between 0 and 0.5. Steel about 0.3, rubber close to 0.5 (nearly incompressible).

A wire stretched within its elastic limit stores energy U equals one half times stress times strain times volume, equivalently one half times Y times strain squared times volume. Per unit volume, the energy density u equals one half times Y times strain squared. NEET asks this in two forms.

In series (same force, total elongation = sum), 1 over Y_eq equals one half of (1 over Y_1 plus 1 over Y_2) for equal cross-sections and lengths. In parallel (same elongation, total force = sum), Y_eq equals (Y_1 plus Y_2) over 2 for equal cross-sections. The detailed formula depends on lengths and areas — be careful in NEET problems.