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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
The Special Theory of Relativity

Concepts Of Physics MCQ Edition [Volume 2] · Physics · The Special Theory of Relativity

The rest distance between Lucknow and Bhopal is 1000 km . A nonstop...

The rest distance between Lucknow and Bhopal is 1000 km1000 \text{ km}. A nonstop train travels at 360 km h−1360 \text{ km h}^{-1}. Determine the distance between Lucknow and Bhopal in the train frame.

Options

  1. A28 nm28 \text{ nm} less than 1000 km1000 \text{ km}
  2. B56 \mum56 \text{ \mu m} less than 1000 km1000 \text{ km}
  3. C112 nm112 \text{ nm} less than 1000 km1000 \text{ km}
  4. D56 nm56 \text{ nm} less than 1000 km1000 \text{ km}

Correct answer

D. 56 nm56 \text{ nm} less than 1000 km1000 \text{ km}

Step-by-step solution

The rest distance is L0=1000 km=106 mL_0 = 1000 \text{ km} = 10^6 \text{ m}.

The speed of the train is v=360 km h−1=360×518 m s−1=100 m s−1v = 360 \text{ km h}^{-1} = 360 \times \dfrac{5}{18} \text{ m s}^{-1} = 100 \text{ m s}^{-1}.

According to the length contraction formula, the distance in the train's frame is L=L01−v2c2L = L_0 \sqrt{1 - \dfrac{v^2}{c^2}}.

Since v≪cv \ll c, using the binomial expansion gives L \approx L_0 \left(1 -\dfrac{v^2}{2c^2}\right).

The decrease in length is ΔL=L0−L=\Delta L = L_0 - L =\dfrac{L_0 v^2}{2c^2}.

Substituting the values:

ΔL=\Delta L =\dfrac{10^6 \times (100)^2}{2 \times (3 \times 10^8)^2}

ΔL=\Delta L =\dfrac{10^{10}}{18 \times 10^{16}}

ΔL=\Delta L =\dfrac{1}{18} \times10−6 m10^{-6} \text{ m}

ΔL≈0.0555×10−6 m=55.5 nm≈56 nm\Delta L \approx 0.0555 \times 10^{-6} \text{ m} = 55.5 \text{ nm} \approx 56 \text{ nm}

Thus, the distance in the train's frame is 56 nm56 \text{ nm} less than 1000 km1000 \text{ km}.
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