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Sound — Question 7

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Question 7

Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in the air and in aluminium to reach the second child.

Answer

Let length of the aluminium rod be l.

Speed of sound in air (vair) at 25°C = 346 ms-1

Speed of sound in aluminium (valu) at 25°C = 6420 ms-1

We know,

Speed = DistanceTime\dfrac{\text{Distance}}{\text{Time}}

or

Time (t) = DistanceSpeed\dfrac{\text{Distance}}{\text{Speed}}

Substituting we get,

tair = l346\dfrac{\text{l}}{\text{346}}

talu = l6420\dfrac{\text{l}}{\text{6420}}

So, the ratio of time taken by sound in air and aluminium is

tairtalu\dfrac{\text{t}_\text{air}}{\text{t}_\text{alu}} = l346l6420\dfrac{\dfrac{\text{l}}{\text{346}}}{\dfrac{\text{l}}{\text{6420}}} = 6420346\dfrac{6420}{346} = 18.55

Hence, the ratio of time taken by sound in air and aluminium = 18.55

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Science | Chapter 11: SoundWeb Content — Quick Reference

Chapter 11: Sound — Quick Reference

longitudinal wave frequency wavelength echo SONAR ultrasound human ear speed of sound

Quick Revision Points

  • Sound: produced by vibration, needs medium, longitudinal wave (compressions + rarefactions)
  • v = fλ; Speed in air ≈ 344 m/s; Solids > Liquids > Gases
  • Loudness → amplitude; Pitch → frequency; Quality → waveform
  • Echo: reflected sound; min distance = 17.2 m (at v = 344 m/s, t ≥ 0.1 s)
  • Audible range: 20 Hz – 20 kHz; Infrasound < 20 Hz; Ultrasound > 20 kHz
  • SONAR: d = vt/2; Human ear: pinna → canal → eardrum → hammer/anvil/stirrup → cochlea → auditory nerve
Exam Tips for Chapter 11
  • Read the detailed chapter notes for complete coverage of all NCERT topics.
  • Practice all NCERT in-text and back exercise questions — they are frequently asked in exams.
  • Focus on comparison tables, diagrams, and definitions — these are high-scoring areas.
  • For numericals (if applicable), practice at least 20 problems of varying difficulty.
  • Refer to the practice question bank (200+ questions) for thorough preparation.
Related Resources
  • Detailed Notes: ch11-sound.html
  • Practice Questions: 100+ questions with answers in 05-practice-questions/
  • Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
  • Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html
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