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Elastic Energy Applications

🎓 Class 11 Physics CBSE Theory Ch 8 – Mechanical Properties of Solids ⏱ ~14 min
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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):

\[F = \frac{YA}{L} \cdot x\]

The work done in stretching the wire by an additional infinitesimal extension \(dx\) is:

\[dW = F \cdot dx = \frac{YA}{L} \cdot x \cdot dx\]

The total work done (and hence energy stored) in stretching the wire from 0 to \(\Delta L\) is:

\[W = \int_0^{\Delta L} \frac{YA}{L} \cdot x \, dx = \frac{YA}{L} \cdot \frac{x^2}{2}\Bigg|_0^{\Delta L} = \frac{YA(\Delta L)^2}{2L}\]

This can be rewritten in several equivalent forms:

Elastic Potential Energy stored in a stretched wire: \[U = \frac{1}{2} \times \text{Stress} \times \text{Strain} \times \text{Volume}\] \[U = \frac{1}{2} \times \frac{F}{A} \times \frac{\Delta L}{L} \times (A \cdot L) = \frac{1}{2} F \cdot \Delta L\] \[U = \frac{1}{2} \times Y \times (\text{Strain})^2 \times \text{Volume}\]
Energy Density (energy stored per unit volume): \[u = \frac{U}{\text{Volume}} = \frac{1}{2} \times \text{Stress} \times \text{Strain} = \frac{1}{2} \times Y \times (\text{Strain})^2 = \frac{(\text{Stress})^2}{2Y}\] Unit: J/m³ = Pa
Extension (ΔL) → Force (F) → Energy = ½F·ΔL (Area of triangle) F ΔL F U = ½ F · ΔL = ½ × stress × strain × V
Force-extension graph for an elastic wire obeying Hooke's law. The elastic energy stored equals the area under the curve (shaded triangle).

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.

Given: \(L = 2.0\) m, \(A = 1.0 \times 10^{-6}\) m², \(\Delta L = 1.0 \times 10^{-3}\) m, \(Y = 2.0 \times 10^{11}\) 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.

Given: Stress \(= 6.0 \times 10^7\) Pa, \(Y = 1.2 \times 10^{11}\) 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.

Solid Beam Compression Tension Neutral axis I-Beam (Efficient) Top flange Bottom flange Web Less material, nearly same strength Much lighter & cheaper!
I-beam cross-section: material is concentrated at the top and bottom flanges where stress is maximum, with a thin web connecting them. This reduces weight without significantly reducing strength.

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.

Given:
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.

Given:
\(b = 0.10\) 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.

Given: \(L = 4.0\) m, \(d = 1.0\) mm \(\Rightarrow r = 5.0 \times 10^{-4}\) m, \(m = 20\) kg
\(F = mg = 200\) 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

Why are pillars thicker at the bottom of buildings? The base supports the weight of all floors above. Since stress = F/A, and the force at the base is maximum, the area must be large to keep stress below the elastic limit. Hence, pillars and columns are wider at the bottom.
Why do railway tracks have expansion gaps? Temperature changes cause tracks to expand or contract. Without gaps, thermal stress could buckle or crack the rails. The elastic energy that would build up due to constrained thermal expansion could be enormous, causing catastrophic failure.
Why does a steel ball bounce better than a clay ball? When a steel ball hits the ground, it deforms elastically — almost all the elastic energy stored during deformation is returned as kinetic energy, making it bounce high. A clay ball deforms plastically — the energy goes into permanent deformation (and heat), so very little is returned as bounce.

Interactive: Material Comparison L4 Analyse

Select two materials to compare their elastic moduli side by side:

vs
Activity — Elastic Energy in Everyday Life L3 Apply
Predict first: If you compress a spring by 2 cm and then by 4 cm, how does the stored energy change? Is it doubled or quadrupled?
  1. 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.
  2. Repeat with double the compression (2 cm). The ball should travel approximately 4 times farther.
  3. This demonstrates that elastic energy depends on the square of the deformation: \(U = \frac{1}{2}kx^2\).
Observation: Doubling the compression quadruples the stored energy (and approximately quadruples the distance the ball travels, assuming all energy converts to kinetic energy).

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

An engineering firm is designing an elevator system. The elevator cabin has a mass of 800 kg and carries a maximum of 6 passengers (average mass 70 kg each). The steel cable has a diameter of 2.0 cm, Young's modulus \(2.0 \times 10^{11}\) Pa, and breaking stress \(8.0 \times 10^{8}\) Pa. The cable length is 50 m.

Q1. L3 Apply Calculate the maximum stress in the cable when fully loaded. Take \(g = 10\) m/s². (2 marks)

Answer:
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)

Answer:
\(\Delta L = \frac{FL}{AY} = \frac{12200 \times 50}{3.14 \times 10^{-4} \times 2.0 \times 10^{11}} = \frac{6.1 \times 10^5}{6.28 \times 10^7} = 9.71 \times 10^{-3}\) m ≈ 9.7 mm.

Q3. L3 Apply Calculate the elastic energy stored in the cable. (2 marks)

Answer:
\(U = \frac{1}{2} F \cdot \Delta L = \frac{1}{2} \times 12200 \times 9.71 \times 10^{-3} = 59.2\) J.

Q4. L4 Analyse What is the safety factor of this cable? Is this adequate for an elevator? (2 marks)

Answer:
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)

Answer:
(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 \(\frac{1}{2} \times Y \times (\text{strain})^2\).

  • A. Both A and R are true, and R is the correct explanation of A.
  • B. Both A and R are true, but R is NOT the correct explanation of A.
  • C. A is true, but R is false.
  • D. A is false, but R is true.
Answer: A. Since \(U = \frac{1}{2} Y \times \text{strain}^2 \times V\), and \(Y\) and \(V\) are constants for a given wire, \(U \propto \text{strain}^2\). R provides the correct formula that explains A.

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.

  • A. Both A and R are true, and R is the correct explanation of A.
  • B. Both A and R are true, but R is NOT the correct explanation of A.
  • C. A is true, but R is false.
  • D. A is false, but R is true.
Answer: A. In bending, the top and bottom layers experience maximum stress while the neutral axis has zero stress. Removing material from near the neutral axis (creating the I-shape) reduces weight without significantly reducing strength. R correctly explains why I-beams are preferred.

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.

  • A. Both A and R are true, and R is the correct explanation of A.
  • B. Both A and R are true, but R is NOT the correct explanation of A.
  • C. A is true, but R is false.
  • D. A is false, but R is true.
Answer: B. A is true — steel deforms and recovers almost perfectly (negligible hysteresis), so nearly all kinetic energy is restored. R is also true, but the higher bounce is not directly caused by higher Y — it is caused by steel's much lower energy loss (lower hysteresis). Rubber loses significant energy as heat during deformation. So R is not the correct explanation of A.

Did You Know?

Frequently Asked Questions - Elastic Energy Applications

What is the main concept covered in Elastic Energy Applications?
In NCERT Class 11 Physics Chapter 8 (Mechanical Properties of Solids), "Elastic Energy Applications" covers core principles and equations needed for board exam success. The MyAiSchool lesson explains the topic with definitions, derivations, worked examples, and interactive simulations. Key formulas and dimensional analysis are included to build conceptual depth and problem-solving skills aligned with the CBSE 2025-26 syllabus.
How is Elastic Energy Applications useful in real-life applications?
Real-life applications of Elastic Energy Applications from NCERT Class 11 Physics Chapter 8 include engineering design, satellite mechanics, sports biomechanics, transportation safety, and electrical/electronic devices. The MyAiSchool lesson links every concept to a tangible example so students see physics as a problem-solving framework for the physical world, not as abstract formulas.
What are the key formulas in Elastic Energy Applications?
Key formulas in Elastic Energy Applications (NCERT Class 11 Physics Chapter 8 Mechanical Properties of Solids) are derived step-by-step in the MyAiSchool lesson. Students should memorize the final formula AND understand its derivation for full board marks. Each formula is listed with its dimensional formula, SI unit, applicability range, and common pitfalls. The Summary section at the end of each part includes a quick-reference formula card.
How does this part connect to other parts of Chapter 8?
NCERT Class 11 Physics Chapter 8 (Mechanical Properties of Solids) is structured so each part builds on the previous one. Elastic Energy Applications connects directly to neighbouring parts via shared definitions, units, and methodology. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Elastic Energy Applications?
CBSE board questions from Elastic Energy Applications typically include: (1) 1-mark MCQs on definitions and formulas, (2) 2-mark short-answer derivations or applications, (3) 3-mark numerical problems with units, (4) 5-mark long-answer derivations followed by application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the Elastic Energy Applications lesson allows students to adjust input parameters (sliders or selectors) and see physical quantities update in real time. To use it effectively: (1) try extreme values to understand limiting cases, (2) compare with the analytical formula, (3) check unit consistency, (4) test special configurations from worked examples. The simulation reinforces conceptual intuition that pure formula manipulation cannot.
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