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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Some Mechanical Properties of Matter

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Some Mechanical Properties of Matter

Consider two wires, A and B , composed of the same material. Wire A...

Consider two wires, AA and BB, composed of the same material. Wire AA possesses a length ll and diameter rr, whereas wire BB has a length 2l2l and diameter r/2r/2. When both wires are stretched by an identical force, the ratio of the elongation in AA to the elongation in BB is

Options

  1. A88
  2. B1/41/4
  3. C44
  4. D1/81/8

Correct answer

D. 1/81/8

Step-by-step solution

Young's modulus is given by Y=FLAΔLY = \dfrac{F L}{A \Delta L}

The elongation of a wire is ΔL=FLAY=4FLπd2Y\Delta L = \dfrac{F L}{A Y} = \dfrac{4 F L}{\pi d^2 Y}

For wire AA, length LA=lL_A = l and diameter dA=rd_A = r. The elongation is:

ΔLA=4Flπr2Y\Delta L_A = \dfrac{4 F l}{\pi r^2 Y}

For wire BB, length LB=2lL_B = 2l and diameter dB=r/2d_B = r/2. The elongation is:

ΔLB=4F(2l)π(r/2)2Y=32Flπr2Y\Delta L_B = \dfrac{4 F (2l)}{\pi (r/2)^2 Y} = \dfrac{32 F l}{\pi r^2 Y}

Taking the ratio of the elongations:

ΔLAΔLB=4Flπr2Y32Flπr2Y=432=18\dfrac{\Delta L_A}{\Delta L_B} = \dfrac{\dfrac{4 F l}{\pi r^2 Y}}{\dfrac{32 F l}{\pi r^2 Y}} = \dfrac{4}{32} = \dfrac{1}{8}
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