Why is steel considered more elastic than rubber in physics? What causes bridges to sag over decades of heavy traffic, and why are structural beams manufactured in an "I" shape rather than solid rectangular blocks? How high can a mountain theoretically grow on Earth before collapsing under its own weight? Mechanical Properties of Solids forms Chapter 9 of the CBSE Class 11 Physics syllabus and is a scoring chapter for JEE Main, JEE Advanced, and NEET.
This comprehensive guide covers Elasticity vs Plasticity, Hooke's Law, complete analysis of the Stress-Strain Curve for metallic wires, Young's Modulus, Shear Modulus, Bulk Modulus, Poisson's Ratio limits, Elastic Potential Energy derivations, engineering applications (I-girders & maximum mountain height), and five step-by-step solved entrance exam numericals.
- 1. Elasticity vs Plasticity, Stress & Strain Types
- 2. Hooke's Law & Complete Stress-Strain Curve Analysis
- 3. Moduli of Elasticity: Young's, Shear & Bulk Modulus
- 4. Poisson's Ratio (ν) & Theoretical/Practical Limits
- 5. Elastic Potential Energy in Stretched Wire & Energy Density
- 6. Engineering Applications: I-Beams & Max Mountain Height
- 7. Solved Entrance Exam Numerical Problems (JEE / NEET)
- 8. Frequently Asked Questions (FAQ)
1. Elasticity vs Plasticity, Stress & Strain Types
When a deforming force acts on a body, internal restoring forces arise to oppose the deformation:
• Plasticity: Property by virtue of which a body undergoes permanent deformation and does not regain its original shape (e.g. Putty, Clay, Paraffin wax).
Types: Longitudinal (Tensile/Compressive), Volumetric ($\Delta P$), Shearing ($\tau = F_t / A$).
• Strain (ε): Fractional change in dimensions: $\epsilon = \frac{\text{Change in Dimension}}{\text{Original Dimension}}$ (Dimensionless!).
Types: Longitudinal ($\Delta L / L$), Volumetric ($\Delta V / V$), Shearing ($\theta = \Delta x / L$).
2. Hooke's Law & Complete Stress-Strain Curve Analysis
\text{Stress} \propto \text{Strain} \implies \text{Stress} = E \times \text{Strain} \implies E = \frac{\text{Stress}}{\text{Strain}}
Where $E$ is the Modulus of Elasticity of the material.
Stress-Strain Curve for a Stretched Metallic Wire
| Region / Point | Curve Feature | Physical Material Behavior |
|---|---|---|
| Region OA (Proportional Limit) | Linear straight line | Hooke's law is strictly obeyed ($\sigma \propto \epsilon$). Wire returns to original length completely. Slope $= Y$. |
| Point B (Yield Point / Elastic Limit) | Upper yield boundary | Maximum stress up to which material regains original shape when load is removed. Stress at B is Yield Strength ($\sigma_y$). |
| Region BC (Plastic Region) | Non-linear curve | Strain increases faster than stress. If load is removed at C, wire retains permanent strain $OO'$ called Permanent Set. |
| Point D (Ultimate Tensile Strength) | Peak maximum stress | Maximum stress material can withstand before necking/thinning begins. Stress at D is Tensile Strength ($\sigma_u$). |
| Point E (Fracture / Breaking Point) | End of curve | Wire breaks/snaps at point E. If distance DE is large ⇒ Ductile (Copper, Steel); If D & E are close ⇒ Brittle (Glass, Cast Iron). |
Consider equal length $L$ and area $A$ of steel and rubber wires subjected to equal stretching force $F$:
$\Delta L_{\text{rubber}} \gg \Delta L_{\text{steel}} \implies Y_{\text{steel}} = \frac{F L}{A \Delta L_{\text{steel}}} \gg Y_{\text{rubber}} = \frac{F L}{A \Delta L_{\text{rubber}}}$.
Since $Y_{\text{steel}} \approx 2 \times 10^{11}\text{ N/m}^2$ while $Y_{\text{rubber}} \approx 10^7\text{ N/m}^2$, Steel is 20,000 times more elastic than rubber!
3. Moduli of Elasticity: Young's, Shear & Bulk Modulus
| Modulus of Elasticity | Definition & Formula | Units & Typical Values |
|---|---|---|
| Young's Modulus ($Y$) | $Y = \frac{\text{Longitudinal Stress}}{\text{Longitudinal Strain}} = \frac{F / A}{\Delta L / L} = \frac{F L}{A \Delta L} = \frac{M g L}{\pi r^2 \Delta L}$ | N/m² or Pa. Steel: $200\text{ GPa}$, Copper: $110\text{ GPa}$, Aluminium: $70\text{ GPa}$. |
| Shear Modulus ($G$ or $\eta$) | $G = \frac{\text{Shearing Stress}}{\text{Shearing Strain}} = \frac{F / A}{\theta} = \frac{F L}{A \Delta x}$ | N/m² or Pa. Typically $G \approx Y/3$ for metals. (Only solids have shear modulus!). |
| Bulk Modulus ($B$) | $B = \frac{\text{Hydraulic Stress}}{\text{Volumetric Strain}} = \frac{-\Delta P}{\Delta V / V} = -V \frac{\Delta P}{\Delta V}$ | N/m² or Pa. Water: $2.2\text{ GPa}$, Air: $1.0 \times 10^{-4}\text{ GPa}$. (Solids > Liquids > Gases). |
| Compressibility ($K$) | $K = \frac{1}{B} = -\frac{1}{V} \frac{\Delta V}{\Delta P}$ | N¯¹·m² or Pa¯¹. Reciprocal of Bulk Modulus. Gases are highly compressible. |
4. Poisson's Ratio (ν) & Theoretical/Practical Limits
When a wire is stretched longitudinally, its length increases while its diameter contracts laterally:
• Theoretical Limits: $-1 \le \nu \le 0.5$.
• Practical Limits for Real Solids: $0 \le \nu \le 0.5$.
• Cork: $\nu \approx 0$ (no lateral expansion when compressed).
• Paraffin Rubber / Liquids: $\nu \approx 0.5$ (Incompressible, constant volume!).
• Steel: $\nu \approx 0.28\text{ to }0.30$.
5. Elastic Potential Energy in Stretched Wire & Energy Density
Work done against internal interatomic restoring forces during wire stretching is stored as Elastic Potential Energy ($U$):
$U = \frac{1}{2} \times \text{Stretching Force} \times \text{Elongation} = \frac{1}{2} F \Delta L$.
Since $F = \frac{Y A \Delta L}{L} \implies$ U = \frac{1}{2} \frac{Y A (\Delta L)^2}{L}.
2. Energy Density ($u = U / \text{Volume}$):
$u = \frac{\frac{1}{2} F \Delta L}{A L} = \frac{1}{2} \left( \frac{F}{A} \right) \left( \frac{\Delta L}{L} \right) = \frac{1}{2} \times \text{Stress} \times \text{Strain}$.
Since $\text{Stress} = Y \times \text{Strain} \implies$ u = \frac{1}{2} Y \epsilon^2 = \frac{\sigma^2}{2Y}.
6. Engineering Applications: I-Beams & Max Mountain Height
δ = (W L³) / (4 Y b d³)
To minimize sag $\delta$:
• Increase depth $d$ (since $\delta \propto 1/d^3$).
• Use I-shaped Girders for bridges & cranes: Provides large depth $d$ and high bending resistance while eliminating excess material from central neutral axis, reducing structural weight and preventing buckling!
$h_{\text{max}} \rho g = \sigma_s \implies h_{\text{max}} = \frac{\sigma_s}{\rho g}$.
With rock density $\rho \approx 3000\text{ kg/m}^3$ and $g = 9.8\text{ m/s}^2$:
$h_{\text{max}} = \frac{3 \times 10^8}{3000 \times 9.8} \approx \mathbf{10\text{ km}}$.
This explains why Mount Everest (8.85 km) is near the maximum possible mountain height on Earth!
7. Solved Entrance Exam Numerical Problems (JEE / NEET)
Area $A = \pi r^2 = 3.1416 \times (10^{-2})^2 = 3.1416 \times 10^{-4}\text{ m}^2$.
(a) Stress $\sigma = \frac{F}{A} = \frac{10^5}{3.1416 \times 10^{-4}} = \mathbf{3.18 \times 10^8\text{ N/m}^2}$.
(b) Elongation $\Delta L = \frac{F L}{A Y} = \frac{10^5 \times 1.0}{3.1416 \times 10^{-4} \times 2 \times 10^{11}} = \frac{10^5}{6.283 \times 10^7} = \mathbf{1.59 \times 10^{-3}\text{ m}} = \mathbf{1.59\text{ mm}}$.
(c) Strain $\epsilon = \frac{\Delta L}{L} = \frac{1.59 \times 10^{-3}}{1.0} = \mathbf{1.59 \times 10^{-3}}$.
Total elongation $\Delta L_{\text{total}} = \Delta L_{\text{copper}} + \Delta L_{\text{steel}} = \frac{F L_c}{A Y_c} + \frac{F L_s}{A Y_s} = \frac{F}{A} \left( \frac{L_c}{Y_c} + \frac{L_s}{Y_s} \right)$.
$\frac{L_c}{Y_c} = \frac{2.2}{1.1 \times 10^{11}} = 2.0 \times 10^{-11}$; $\frac{L_s}{Y_s} = \frac{1.6}{2.0 \times 10^{11}} = 0.8 \times 10^{-11}$.
$\Delta L_{\text{total}} = \frac{F}{7.07 \times 10^{-6}} \times (2.8 \times 10^{-11}) = F \times 3.96 \times 10^{-6}\text{ m}$.
$0.70 \times 10^{-3} = F \times 3.96 \times 10^{-6} \implies F = \frac{0.70 \times 10^{-3}}{3.96 \times 10^{-6}} = \mathbf{176.8\text{ N}}$.
Work done formula: $W = \frac{1}{2} \frac{Y A (\Delta L)^2}{L}$.
$W = \frac{1}{2} \times \frac{2 \times 10^{11} \times 10^{-6} \times (10^{-3})^2}{2} = \frac{2 \times 10^5 \times 10^{-6}}{4} = \frac{0.2}{4} = \mathbf{0.05\text{ Joules}}$.
Bulk modulus formula: $B = \frac{\Delta P}{\Delta V / V} \implies \frac{\Delta V}{V} = \frac{\Delta P}{B}$.
$\frac{\Delta V}{V} = \frac{3.0 \times 10^7}{2.2 \times 10^9} = \mathbf{1.36 \times 10^{-2}} = \mathbf{1.36\%}$.
Poisson's ratio formula: $\nu = \frac{\Delta d / d}{\Delta L / L} \implies \frac{\Delta d}{d} = \nu \times \frac{\Delta L}{L}$.
$\Delta d = \nu \times d \times \frac{\Delta L}{L} = 0.3 \times 5\text{ mm} \times \frac{2 \times 10^{-3}}{4} = 1.5 \times 0.5 \times 10^{-3}\text{ mm} = \mathbf{7.5 \times 10^{-4}\text{ mm}} = \mathbf{0.75\text{ \mu m}}$.
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8. Frequently Asked Questions (FAQ)
Because Young's Modulus measures resistance to deformation ($Y = \text{Stress}/\text{Strain}$). Steel requires a vastly larger stress for the same strain than rubber ($Y_{\text{steel}} = 2 \times 10^{11}\text{ N/m}^2$ vs $Y_{\text{rubber}} = 10^7\text{ N/m}^2$), making steel 20,000 times more elastic.
The theoretical limits for Poisson's ratio are $-1 \le \nu \le 0.5$. However, for all practical isotropic solid materials, Poisson's ratio ranges between $0 \le \nu \le 0.5$.
Bending depression $\delta = (WL^3)/(4Ybd^3)$ depends inversely on depth cubed ($1/d^3$). An I-shaped girder provides large depth $d$ and high resistance to bending while eliminating material from the central neutral axis, reducing structural weight and preventing side-buckling.
The total elastic potential energy is $U = \frac{1}{2} F \Delta L = \frac{1}{2} \frac{Y A (\Delta L)^2}{L}$, and energy per unit volume (energy density) is $u = \frac{1}{2} \times \text{Stress} \times \text{Strain} = \frac{1}{2} Y \epsilon^2$.
The mountain's base rock experiences pressure $P = h \rho g$. If $P$ exceeds the ultimate shearing strength of rock ($\sigma_s \approx 3 \times 10^8\text{ N/m}^2$), the base rock yields and flows plastically, limiting $h_{\text{max}} = \sigma_s / (\rho g) \approx 10\text{ km}$.
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