This MCQ module is based on: Elastic Moduli
Elastic Moduli
This assessment will be based on: Elastic Moduli
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Elastic Moduli
8.4 Young's Modulus (Y)
The ratio of longitudinal (tensile or compressive) stress to longitudinal strain is called Young's modulus, denoted by \(Y\):
Young's modulus applies only to solids — fluids do not have a definite shape to resist longitudinal deformation.
Table 8.1 — Young's Modulus of Common Materials
| Material | Young's Modulus Y (GPa) |
|---|---|
| Steel | 200 |
| Copper | 120 |
| Aluminium | 70 |
| Brass | 91 |
| Iron (wrought) | 190 |
| Glass | 65 |
| Bone | 9 – 16 |
| Rubber | 0.01 – 0.10 |
| Tungsten | 410 |
Worked Examples — Young's Modulus
Example 1 (NCERT 8.1): Steel Beam Elongation
A structural steel rod has a radius of 10 mm and a length of 1.0 m. A force of \(100 \times 10^3\) N (100 kN) stretches it along its length. Calculate the elongation. Given: \(Y_{\text{steel}} = 2.0 \times 10^{11}\) Pa.
\(r = 10\) mm \(= 0.01\) m, \(L = 1.0\) m, \(F = 100 \times 10^3 = 10^5\) N
\(Y = 2.0 \times 10^{11}\) Pa
Step 1: Cross-sectional area \[A = \pi r^2 = \pi \times (0.01)^2 = \pi \times 10^{-4} = 3.14 \times 10^{-4} \text{ m}^2\] Step 2: Using \(Y = \frac{FL}{A \cdot \Delta L}\), solve for \(\Delta L\): \[\Delta L = \frac{FL}{AY} = \frac{10^5 \times 1.0}{3.14 \times 10^{-4} \times 2.0 \times 10^{11}}\] \[\Delta L = \frac{10^5}{6.28 \times 10^{7}} = 1.59 \times 10^{-3} \text{ m} \approx 1.59 \text{ mm}\] Dimensional check: \(\frac{[\text{N}][\text{m}]}{[\text{m}^2][\text{Pa}]} = \frac{[\text{N}\cdot\text{m}]}{[\text{m}^2]\cdot[\text{N/m}^2]} = [\text{m}]\) ✓
Answer: The elongation is \(\boxed{1.59 \text{ mm}}\).
Example 2: Copper Wire Under Load
A copper wire of length 3.0 m and diameter 0.80 mm is stretched by a force of 50 N. Find the extension produced. Given: \(Y_{\text{Cu}} = 1.2 \times 10^{11}\) Pa.
\(L = 3.0\) m, \(d = 0.80\) mm \(\Rightarrow r = 0.40\) mm \(= 4.0 \times 10^{-4}\) m
\(F = 50\) N, \(Y = 1.2 \times 10^{11}\) Pa
Step 1: \(A = \pi r^2 = \pi \times (4.0 \times 10^{-4})^2 = \pi \times 1.6 \times 10^{-7} = 5.027 \times 10^{-7}\) m²
Step 2: \[\Delta L = \frac{FL}{AY} = \frac{50 \times 3.0}{5.027 \times 10^{-7} \times 1.2 \times 10^{11}} = \frac{150}{6.032 \times 10^{4}} = 2.49 \times 10^{-3} \text{ m}\] Answer: \(\Delta L = \boxed{2.49 \text{ mm}}\)
Example 3: Comparing Two Wires
Two wires of the same material have lengths in the ratio 1:2 and diameters in the ratio 2:1. If the same force is applied to both, find the ratio of their extensions.
Since the material is the same, \(Y\) is the same for both.
\[\Delta L = \frac{FL}{AY} = \frac{FL}{\pi r^2 Y} = \frac{FL}{\pi (d/2)^2 Y} = \frac{4FL}{\pi d^2 Y}\] For Wire 1: \(\Delta L_1 = \frac{4F \cdot L}{\pi (2d)^2 Y} = \frac{4FL}{4\pi d^2 Y} = \frac{FL}{\pi d^2 Y}\)
For Wire 2: \(\Delta L_2 = \frac{4F \cdot 2L}{\pi d^2 Y} = \frac{8FL}{\pi d^2 Y}\)
\[\frac{\Delta L_1}{\Delta L_2} = \frac{FL/(\pi d^2 Y)}{8FL/(\pi d^2 Y)} = \frac{1}{8}\] Answer: The ratio of extensions is \(\boxed{1:8}\). The thinner, longer wire stretches 8 times more.
8.5 Bulk Modulus (B)
When a body is subjected to uniform pressure from all sides (hydraulic stress), its volume changes. The bulk modulus relates the applied pressure to the fractional volume change.
SI unit: Pa. Applies to solids, liquids, and gases.
| Material | Bulk Modulus B (GPa) |
|---|---|
| Steel | 160 |
| Copper | 140 |
| Aluminium | 72 |
| Glass | 37 |
| Water | 2.2 |
| Air (at STP) | 1.0 × 10¹&sup4; Pa |
Example 4: Volume Change Under Pressure
A solid copper sphere of volume \(0.50\) m³ is placed at the bottom of a lake where the pressure is \(2.0 \times 10^{6}\) Pa above atmospheric pressure. Calculate the change in volume. Given: \(B_{\text{Cu}} = 140\) GPa.
Using: \(B = \frac{-pV}{\Delta V}\) so \(\Delta V = \frac{-pV}{B}\) \[\Delta V = \frac{-2.0 \times 10^{6} \times 0.50}{140 \times 10^{9}} = \frac{-1.0 \times 10^{6}}{1.4 \times 10^{11}} = -7.14 \times 10^{-6} \text{ m}^3\] The negative sign indicates a decrease in volume.
Answer: The volume decreases by \(\boxed{7.14 \times 10^{-6} \text{ m}^3}\) or about \(7.14\) cm³.
8.5.3 Shear Modulus / Modulus of Rigidity (G)
The shear modulus (also called modulus of rigidity) relates the tangential stress to the angular deformation (shearing strain).
| Material | Shear Modulus G (GPa) |
|---|---|
| Steel | 84 |
| Copper | 42 |
| Aluminium | 25 |
| Iron | 70 |
| Glass | 23 |
| Lead | 5.6 |
Example 5: Metal Cube Under Shear
A metal cube of side 10 cm is subjected to a tangential force of \(5.0 \times 10^4\) N on its top face while the bottom face is fixed. If the shear modulus of the metal is 80 GPa, find the lateral displacement of the top face.
Side \(L = 10\) cm \(= 0.10\) m, \(F = 5.0 \times 10^4\) N
Area of top face: \(A = (0.10)^2 = 0.01\) m²
\(G = 80 \times 10^9\) Pa
Using: \(G = \frac{F/A}{\Delta x / L}\), so \(\Delta x = \frac{FL}{AG}\) \[\Delta x = \frac{5.0 \times 10^4 \times 0.10}{0.01 \times 80 \times 10^9} = \frac{5.0 \times 10^3}{8.0 \times 10^8} = 6.25 \times 10^{-6} \text{ m}\] Answer: The top face is displaced by \(\boxed{6.25 \,\mu\text{m}}\) (about 6 micrometres).
Example 6: Comparing Shear and Tensile Stress
A steel rod of diameter 2.0 cm and length 50 cm is subjected to: (a) a tensile force of 10 kN, (b) a shear force of 10 kN. Calculate the deformation in each case. Given: \(Y = 200\) GPa, \(G = 84\) GPa.
\(A = \pi r^2 = \pi \times (0.01)^2 = 3.14 \times 10^{-4}\) m²
(a) Tensile elongation: \[\Delta L = \frac{FL}{AY} = \frac{10^4 \times 0.50}{3.14 \times 10^{-4} \times 200 \times 10^9} = \frac{5 \times 10^3}{6.28 \times 10^7} = 7.96 \times 10^{-5} \text{ m}\] \(\Delta L \approx 0.080\) mm
(b) Shear displacement: \[\Delta x = \frac{FL}{AG} = \frac{10^4 \times 0.50}{3.14 \times 10^{-4} \times 84 \times 10^9} = \frac{5 \times 10^3}{2.64 \times 10^7} = 1.89 \times 10^{-4} \text{ m}\] \(\Delta x \approx 0.189\) mm
Answer: (a) Tensile: \(\boxed{0.080 \text{ mm}}\), (b) Shear: \(\boxed{0.189 \text{ mm}}\). Shear deformation is larger because \(G < Y\).
8.5.4 Poisson's Ratio (σ)
When a wire is stretched along its length, it becomes thinner — its diameter decreases. This lateral contraction accompanies every longitudinal extension. Poisson's ratio quantifies this effect.
Range: Theoretically, \(-1 \leq \sigma \leq 0.5\). For most materials, \(0 < \sigma < 0.5\).
| Material | Poisson's Ratio (σ) |
|---|---|
| Steel | 0.28 |
| Copper | 0.33 |
| Aluminium | 0.33 |
| Glass | 0.24 |
| Rubber | 0.49 |
| Cork | ≈ 0 |
- Take a wide rubber band and mark two dots across its width.
- Stretch the band along its length and observe the distance between the dots — it decreases.
- Now try pushing a rubber stopper into a bottle — it resists because it expands laterally.
- Try the same with a cork stopper — it slides in easily because it barely expands sideways.
Key Takeaway: Poisson's ratio determines whether a material is suitable for applications involving compression into tight spaces.
Competency-Based Questions
Q1. L1 Remember Which elastic modulus determines a material's resistance to longitudinal compression?
Q2. L3 Apply What is the minimum Young's modulus required for the column material? (3 marks)
\(Y = \frac{FL}{A \cdot \Delta L} = \frac{5.0 \times 10^5 \times 3.0}{0.02 \times 0.50 \times 10^{-3}} = \frac{1.5 \times 10^6}{1.0 \times 10^{-5}} = 1.5 \times 10^{11}\) Pa = 150 GPa.
Steel (Y = 200 GPa) or iron (Y = 190 GPa) would work. Aluminium (70 GPa) would not.
Q3. L2 Understand Explain why bulk modulus applies to all states of matter but shear modulus applies only to solids. (2 marks)
Q4. L4 Analyse A steel wire and a copper wire have the same length and diameter. The same tensile force is applied to both. Which wire elongates more and by what factor? Use: \(Y_{\text{steel}} = 200\) GPa, \(Y_{\text{Cu}} = 120\) GPa. (3 marks)
\(\Delta L = \frac{FL}{AY}\). Since \(F\), \(L\), and \(A\) are the same:
\(\frac{\Delta L_{\text{Cu}}}{\Delta L_{\text{steel}}} = \frac{Y_{\text{steel}}}{Y_{\text{Cu}}} = \frac{200}{120} = \frac{5}{3} \approx 1.67\)
The copper wire elongates \(\frac{5}{3}\) times more than the steel wire.
Q5. L5 Evaluate Cork has Poisson's ratio close to zero. Explain why this makes it the ideal material for sealing wine bottles. What problems would arise if rubber (σ ≈ 0.49) were used instead? (3 marks)
Assertion-Reason Questions
Assertion (A): Young's modulus is defined only for solids, not for liquids or gases.
Reason (R): Liquids and gases do not have a fixed shape, so the concept of longitudinal strain is not applicable to them.
Assertion (A): The bulk modulus of a gas is much smaller than that of a solid.
Reason (R): Gas molecules are far apart and easily compressed, whereas solid molecules are closely packed and strongly resist compression.
Assertion (A): For most materials, Young's modulus is greater than the shear modulus.
Reason (R): It is easier to change the shape of a body than to change its length.