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Sound Waves: Characteristics and Applications: CBSE Class 9 Physics practice questions

Practise 50 questions on Sound Waves: Characteristics and Applications, free, with an explanation for each answer.

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Sample questions

In a science-fiction film, a spacecraft explodes far out in space. The astronaut watching sees the flash and hears a loud bang at the same moment. What is wrong with this scene?

Animation of an astronaut watching a spacecraft explode in space
  1. Nothing is wrong: light and sound both travel through space at the same speed
  2. The bang should be heard first, because sound travels faster than light
  3. No bang should be heard at all, because space is almost a vacuum and sound needs a medium
  4. The bang should be heard a few seconds after the flash, as thunder follows lightning
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C. No bang should be heard at all, because space is almost a vacuum and sound needs a medium

Outer space is almost a vacuum, and sound is a mechanical wave, so it cannot travel there. The astronaut would see the flash, because light needs no medium, but would hear nothing. Option d would be correct on the Earth, where sound travels through air far more slowly than light. Astronauts on a spacewalk talk to each other through special communication devices fitted in their spacesuits.

A friend pushes and pulls the end of a slinky steadily. You count 36 compressions passing the flag P in 3 s. What is the frequency of the wave?

Animation of a slinky with compressions travelling past a flag marked P
  1. 108 Hz108\ \text{Hz}
  2. 12 Hz12\ \text{Hz}
  3. 0.083 Hz0.083\ \text{Hz}
  4. 36 Hz36\ \text{Hz}
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B. 12 Hz12\ \text{Hz}

Each compression passing P is one oscillation, so ν=number of oscillationstime=363 s=12 Hz\nu = \dfrac{\text{number of oscillations}}{\text{time}} = \dfrac{36}{3\ \text{s}} = 12\ \text{Hz} 108 Hz comes from multiplying instead of dividing. 0.083 Hz is 336\tfrac{3}{36}, the ratio turned upside down (that is actually the time period in seconds), and 36 Hz ignores the 3 s.

Gunjan presses her ear to a long steel fence. Her friend knocks on the fence 850 m away. Sound travels at 5000 m s−15000\ \text{m s}^{-1} in steel and 340 m s−1340\ \text{m s}^{-1} in air, so Gunjan hears two knocks. What is the time gap between them, and can she hear them as separate sounds? (Two sounds must be at least 0.1 s apart to be heard separately.)

Animation of a knock travelling quickly along a steel fence and more slowly through the air
  1. 2.33 s; yes, she hears two separate knocks
  2. 2.67 s; yes, she hears two separate knocks
  3. 0.17 s; yes, she hears two separate knocks
  4. 2.33 s; no, the gap is too short to notice
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A. 2.33 s; yes, she hears two separate knocks

Through air: 850340=2.5 s\dfrac{850}{340} = 2.5\ \text{s}. Through steel: 8505000=0.17 s\dfrac{850}{5000} = 0.17\ \text{s}. Gap =2.5−0.17=2.33 s= 2.5 - 0.17 = 2.33\ \text{s}, far more than 0.1 s, so she hears the knock twice: first through the steel, then through the air. 2.67 s adds the times instead of subtracting, and 0.17 s is only the steel time.

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