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JEE Main 20139 AprilOnline
Exam
JEE Main
Year
2013
Date
9 April
Mode
Online
Subject
Physics
Chapter
Waves and Sound

JEE Main 2013 · 9 April · Online · Physics · Waves and Sound

An engine approaches a hill with a constant speed. When it is at a...

An engine approaches a hill with a constant speed. When it is at a distance of 0.9 km0.9 \mathrm{~km}, it blows a whistle whose echo is heard by the driver after 55 seconds. If the speed of sound in air is 330 m/s330 \mathrm{~m} / \mathrm{s}, then the speed of the engine is :An engine approaches a hill with a constant speed. When it is at a distance of 0.9 ~km , it blows a whistle whose echo

Options

  1. A32 m/s32 \mathrm{~m} / \mathrm{s}
  2. B27.5 m/s27.5 \mathrm{~m} / \mathrm{s}
  3. C60 m/s60 \mathrm{~m} / \mathrm{s}
  4. D30 m/s30 \mathrm{~m} / \mathrm{s}

Correct answer

D. 30 m/s30 \mathrm{~m} / \mathrm{s}

Step-by-step solution

An engine approaches a hill with a constant speed. When it is at a distance of 0.9 ~km , it blows a whistle whose echo
Let after 5 sec5 \mathrm{~sec} engine at point C\mathrm{C} t=AB330+BC3305=0.9×1000330+BC330∴BC=750 m\begin{aligned} & \mathrm{t}=\frac{\mathrm{AB}}{330}+\frac{\mathrm{BC}}{330} \\ & 5=\frac{0.9 \times 1000}{330}+\frac{\mathrm{BC}}{330} \\ & \therefore \mathrm{BC}=750 \mathrm{~m} \end{aligned} Distance travelled by engine in 5 sec5 \mathrm{~sec} =900 m−750 m=150 m=900 \mathrm{~m}-750 \mathrm{~m}=150 \mathrm{~m} Therefore velocity of engine =150 m5 sec=30 m/s=\frac{150 \mathrm{~m}}{5 \mathrm{~sec}}=30 \mathrm{~m} / \mathrm{s}
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