This MCQ module is based on: Elastic Energy Applications
Elastic Energy Applications
This assessment will be based on: Elastic Energy Applications
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Elastic Energy Applications
8.5.5 Elastic Potential Energy
When an external force stretches a wire (or deforms any elastic body), work is done against the internal restoring forces. This work is stored as elastic potential energy in the deformed body.
Derivation of Elastic Energy
Consider a wire of original length \(L\) and cross-sectional area \(A\) being stretched by a force \(F\). At any instant when the extension is \(x\), the internal restoring force equals the applied force (for equilibrium):
The work done in stretching the wire by an additional infinitesimal extension \(dx\) is:
The total work done (and hence energy stored) in stretching the wire from 0 to \(\Delta L\) is:
This can be rewritten in several equivalent forms:
Worked Examples — Elastic Energy
Example 1: Energy Stored in a Stretched Wire
A steel wire of length 2.0 m and cross-sectional area \(1.0 \times 10^{-6}\) m² is stretched by 1.0 mm. Calculate the elastic energy stored. Given: \(Y = 2.0 \times 10^{11}\) Pa.
Method 1: Using \(U = \frac{YA(\Delta L)^2}{2L}\) \[U = \frac{2.0 \times 10^{11} \times 1.0 \times 10^{-6} \times (1.0 \times 10^{-3})^2}{2 \times 2.0}\] \[U = \frac{2.0 \times 10^{11} \times 1.0 \times 10^{-6} \times 1.0 \times 10^{-6}}{4.0} = \frac{2.0 \times 10^{-1}}{4.0} = 0.05 \text{ J}\] Method 2: Using \(U = \frac{1}{2} \times Y \times (\text{strain})^2 \times \text{Volume}\)
Strain = \(\frac{1.0 \times 10^{-3}}{2.0} = 5.0 \times 10^{-4}\)
Volume = \(A \times L = 1.0 \times 10^{-6} \times 2.0 = 2.0 \times 10^{-6}\) m³
\[U = \frac{1}{2} \times 2.0 \times 10^{11} \times (5.0 \times 10^{-4})^2 \times 2.0 \times 10^{-6}\] \[= \frac{1}{2} \times 2.0 \times 10^{11} \times 2.5 \times 10^{-7} \times 2.0 \times 10^{-6} = 0.05 \text{ J}\] Answer: \(\boxed{U = 0.05 \text{ J} = 50 \text{ mJ}}\)
Example 2: Energy Density
Calculate the energy density in a copper wire subjected to a stress of \(6.0 \times 10^7\) Pa. Given: \(Y_{\text{Cu}} = 1.2 \times 10^{11}\) Pa.
Using: \(u = \frac{(\text{Stress})^2}{2Y}\) \[u = \frac{(6.0 \times 10^7)^2}{2 \times 1.2 \times 10^{11}} = \frac{3.6 \times 10^{15}}{2.4 \times 10^{11}} = 1.5 \times 10^{4} \text{ J/m}^3\] Dimensional check: \(\frac{[\text{Pa}]^2}{[\text{Pa}]} = [\text{Pa}] = [\text{J/m}^3]\) ✓
Answer: Energy density = \(\boxed{1.5 \times 10^4 \text{ J/m}^3}\)
8.6 Applications of Elastic Behaviour of Solids
Understanding how materials deform under stress is crucial for engineering design. Here are key real-world applications discussed in the NCERT text:
Why Do Beams Have an I-Shaped Cross-Section?
When a beam is loaded at its centre (while supported at the ends), the top layers are compressed and the bottom layers are stretched. The middle layers (near the neutral surface) experience almost no stress. Since the middle contributes little to load-bearing, removing material there and concentrating it at the top and bottom flanges creates an I-beam that is much lighter yet almost as strong as a solid rectangular beam.
Crane Ropes: Maximum Load from Breaking Stress
In crane and elevator design, engineers must ensure the rope can support the load without breaking. The key calculation involves the breaking stress (ultimate tensile strength) of the rope material.
Example 3 (NCERT 8.2): Minimum Rope Diameter
A crane is designed to lift loads up to \(10^4\) kg. The steel rope used has a breaking stress of \(5.0 \times 10^8\) Pa. Calculate the minimum diameter of the rope required, using a safety factor of 10. Take \(g = 9.8\) m/s².
Maximum mass: \(m = 10^4\) kg
Breaking stress: \(\sigma_b = 5.0 \times 10^8\) Pa
Safety factor: \(n = 10\)
Step 1: Maximum working stress
The safe working stress = Breaking stress / Safety factor \[\sigma_{\text{safe}} = \frac{5.0 \times 10^8}{10} = 5.0 \times 10^7 \text{ Pa}\] Step 2: Required area
Force: \(F = mg = 10^4 \times 9.8 = 9.8 \times 10^4\) N \[A = \frac{F}{\sigma_{\text{safe}}} = \frac{9.8 \times 10^4}{5.0 \times 10^7} = 1.96 \times 10^{-3} \text{ m}^2\] Step 3: Diameter \[A = \pi r^2 = \frac{\pi d^2}{4} \implies d = \sqrt{\frac{4A}{\pi}} = \sqrt{\frac{4 \times 1.96 \times 10^{-3}}{3.14}}\] \[d = \sqrt{2.497 \times 10^{-3}} = 0.05 \text{ m} = 5.0 \text{ cm}\] Answer: The minimum diameter is \(\boxed{d \approx 5.0 \text{ cm}}\). In practice, an even thicker rope would be used to account for dynamic loads, wear, and fatigue.
Example 4: Bridge Beam Deflection
A steel beam of rectangular cross-section (width 10 cm, depth 20 cm) and length 4.0 m is used in a bridge. A concentrated load of \(2.0 \times 10^4\) N acts at its centre. Estimate the maximum deflection. Given: \(Y = 2.0 \times 10^{11}\) Pa. The deflection formula for a simply supported beam with a central load is \(\delta = \frac{WL^3}{48YI}\), where \(I = \frac{bd^3}{12}\) is the second moment of area.
\(b = 0.10\) m, \(d = 0.20\) m, \(L = 4.0\) m
\(W = 2.0 \times 10^4\) N, \(Y = 2.0 \times 10^{11}\) Pa
Step 1: Second moment of area \[I = \frac{bd^3}{12} = \frac{0.10 \times (0.20)^3}{12} = \frac{0.10 \times 8.0 \times 10^{-3}}{12} = 6.67 \times 10^{-5} \text{ m}^4\] Step 2: Deflection \[\delta = \frac{WL^3}{48YI} = \frac{2.0 \times 10^4 \times (4.0)^3}{48 \times 2.0 \times 10^{11} \times 6.67 \times 10^{-5}}\] \[= \frac{2.0 \times 10^4 \times 64}{48 \times 2.0 \times 10^{11} \times 6.67 \times 10^{-5}} = \frac{1.28 \times 10^6}{6.40 \times 10^8} = 2.0 \times 10^{-3} \text{ m}\] Answer: Maximum deflection \(\delta = \boxed{2.0 \text{ mm}}\). This is well within acceptable limits for a bridge beam.
Example 5: Energy in a Loaded Wire
A wire of length 4.0 m and diameter 1.0 mm supports a load of 20 kg. Calculate: (a) the extension, (b) the elastic energy stored, (c) the energy density. Take \(Y = 2.0 \times 10^{11}\) Pa, \(g = 10\) m/s².
\(F = mg = 200\) N, \(A = \pi r^2 = \pi \times 2.5 \times 10^{-7} = 7.854 \times 10^{-7}\) m²
(a) Extension: \[\Delta L = \frac{FL}{AY} = \frac{200 \times 4.0}{7.854 \times 10^{-7} \times 2.0 \times 10^{11}} = \frac{800}{1.571 \times 10^5} = 5.09 \times 10^{-3} \text{ m}\] \(\Delta L \approx 5.1\) mm
(b) Elastic energy: \[U = \frac{1}{2} F \cdot \Delta L = \frac{1}{2} \times 200 \times 5.09 \times 10^{-3} = 0.509 \text{ J}\] (c) Energy density:
Volume \(= A \times L = 7.854 \times 10^{-7} \times 4.0 = 3.14 \times 10^{-6}\) m³ \[u = \frac{U}{V} = \frac{0.509}{3.14 \times 10^{-6}} = 1.62 \times 10^5 \text{ J/m}^3\] Answers: (a) \(\boxed{5.1 \text{ mm}}\), (b) \(\boxed{0.51 \text{ J}}\), (c) \(\boxed{1.62 \times 10^5 \text{ J/m}^3}\)
Real-World Observations Explained
Interactive: Material Comparison L4 Analyse
Select two materials to compare their elastic moduli side by side:
- Take a spring and compress it by a measured amount (say 1 cm) using a ruler. Release it against a lightweight ball and measure how far the ball travels.
- Repeat with double the compression (2 cm). The ball should travel approximately 4 times farther.
- This demonstrates that elastic energy depends on the square of the deformation: \(U = \frac{1}{2}kx^2\).
Conclusion: Since \(U \propto (\text{deformation})^2\), even a small increase in strain produces a disproportionately large increase in stored elastic energy. This is why overstretching a wire slightly beyond its limits can cause sudden, violent fracture.
Competency-Based Questions
Q1. L3 Apply Calculate the maximum stress in the cable when fully loaded. Take \(g = 10\) m/s². (2 marks)
Total mass = 800 + 6(70) = 1220 kg. Force = 1220 × 10 = 12200 N.
\(A = \pi r^2 = \pi \times (0.01)^2 = 3.14 \times 10^{-4}\) m²
Stress = 12200 / (3.14 × 10¹&sup4;) = 3.88 × 10&sup7; Pa ≈ 38.8 MPa.
Q2. L3 Apply What is the extension of the cable under this full load? (2 marks)
\(\Delta L = \frac{FL}{AY} = \frac{12200 \times 50}{3.14 \times 10^{-4} \times 2.0 \times 10^{11}} = \frac{6.1 \times 10^5}{6.28 \times 10^7} = 9.71 \times 10^{-3}\) m ≈ 9.7 mm.
Q3. L3 Apply Calculate the elastic energy stored in the cable. (2 marks)
\(U = \frac{1}{2} F \cdot \Delta L = \frac{1}{2} \times 12200 \times 9.71 \times 10^{-3} = 59.2\) J.
Q4. L4 Analyse What is the safety factor of this cable? Is this adequate for an elevator? (2 marks)
Safety factor = Breaking stress / Working stress = (8.0 × 10&sup8;) / (3.88 × 10&sup7;) ≈ 20.6.
A safety factor of about 20 is excellent for an elevator — typically a minimum of 8-10 is required. This cable is well within safe limits.
Q5. L5 Evaluate Explain why elevator cables are not made of rubber, even though rubber is more elastic (in the common-sense meaning) than steel. (3 marks)
(i) Rubber has a very low Young's modulus (~0.01 GPa vs steel's 200 GPa), so a rubber cable would stretch enormously under load — passengers would experience huge vertical displacement.
(ii) Rubber has much lower ultimate tensile strength, so a much thicker cable would be needed.
(iii) Rubber suffers from elastic hysteresis — energy is lost as heat in every loading-unloading cycle. Over many cycles, the cable would overheat and degrade.
(iv) In physics, steel is actually "more elastic" than rubber — it returns to its original shape more precisely and with less energy loss.
Assertion-Reason Questions
Assertion (A): The elastic energy stored in a wire is proportional to the square of the strain.
Reason (R): Elastic energy per unit volume equals \(\frac{1}{2} \times Y \times (\text{strain})^2\).
Assertion (A): I-beams are preferred over solid rectangular beams in construction.
Reason (R): In a loaded beam, the layers near the neutral axis contribute very little to load-bearing capacity.
Assertion (A): A steel ball bounces higher than a rubber ball when dropped from the same height onto a hard surface.
Reason (R): Steel has a much higher Young's modulus than rubber.