Stress = Restoring force per unit area: σ = F/A

Types:
• Tensile/Compressive stress (normal to area)
• Shear stress (tangential to area)

Strain = Fractional deformation (dimensionless)

• Longitudinal strain = ΔL/L
• Volumetric strain = ΔV/V
• Shear strain = Δx/L = tanθ ≈ θ (for small angles)

Points on curve:
A: Proportional limit (Hooke's law valid)
B: Elastic limit (up to B, elastic)
C: Yield point (plastic deformation starts)
D: Ultimate tensile strength
E: Fracture point

💡 NEET TIPS & SHORTCUTS

• For a wire under load, Young's modulus is constant for a material, independent of dimensions.
• Bulk modulus of a gas is isothermal (B = P) or adiabatic (B = γP).
• Shear modulus for liquids is zero (they cannot sustain shear).
• Breaking stress is constant for a material, not breaking force.

⚠️ COMMON MISTAKES

• Confusing stress with pressure (pressure is isotropic, stress is directional).
• Using ΔL = FL/AY without checking elastic limit.
• Forgetting that strain is dimensionless.
• Assuming Poisson's ratio is always 0.5 for all materials.

Young's modulus: Y = FL/(AΔL)

Bulk modulus: B = –V ΔP/ΔV

Shear modulus: G = FL/(AΔx)

Energy density: u = ½ σ ε = ½ Y ε²

Poisson's ratio: σ = –(lateral strain)/(longitudinal strain)

Relation: Y = 3B(1 – 2σ), Y = 2G(1 + σ)