08Class 11 Physics

Mechanical Properties of Solids formulas

Stress, strain and elastic moduli formulas for Mechanical Properties of Solids. A short sheet, and a common source of quick numericals.

Stress and strain

Stress
σ=FA\displaystyle \sigma = \frac{F}{A}
Longitudinal strain
ε=ΔLL\displaystyle \varepsilon = \frac{\Delta L}{L}
Hooke's law
stress=modulus×strain\displaystyle \text{stress} = \text{modulus} \times \text{strain}

Within the elastic limit.

Moduli of elasticity

Young's modulus
Y=FLA ΔL\displaystyle Y = \frac{FL}{A\,\Delta L}
Shear modulus
G=F/AΔx/L=F/Aθ\displaystyle G = \frac{F/A}{\Delta x / L} = \frac{F/A}{\theta}
Bulk modulus
B=−pΔV/V\displaystyle B = -\frac{p}{\Delta V / V}
Compressibility
k=1B\displaystyle k = \frac{1}{B}
Elastic energy per unit volume
u=12×stress×strain\displaystyle u = \tfrac12 \times \text{stress} \times \text{strain}

Where students slip up

  • Treating strain as having units. It's a ratio, so stress and modulus share the same units.
  • Forgetting the area is πr² for a wire, or that doubling a radius quadruples the area.
  • Using the cross-section's length in shear. In shear, the force is parallel to the area it acts on.

Try it on your weakest chapter.

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