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JEE Main 2013 (9 Apr, Shift 1) · Reflection, Superposition & Interference

Reflection, Superposition & Interference question from JEE Main 2013 (9 Apr, Shift 1)

Q19JEE Main 2013 · 9 APR · SHIFT 1Medium

Reflection, Superposition & Interference

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 is heard by the driver after 5 seconds. If the speed of sound in air is 330 m/s, then the speed of the engine is:

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Answer: (d)

  1. In 5 s5\text{ s} the engine moves 5v5v, so it is at 900−5v900 - 5v from the hill when it hears the echo.
  2. Sound path: 900+(900−5v)=330×5=1650900 + (900 - 5v) = 330 \times 5 = 1650.
  3. 1800−5v=1650⇒v=30 m/s1800 - 5v = 1650 \Rightarrow v = 30\text{ m/s}.
  4. So the answer is (d) 30 m/s30\text{ m/s}.
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