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CBSE Class 8 Science Question 5 of 13

Sound — Question 5

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Question 5

A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer

Given,

Total number of oscillations = 40

Total time taken = 4 s

We know,

Frequency = Total number of oscillationsTotal time taken\dfrac{\text{Total number of oscillations}}{\text{Total time taken}}

Substituting we get,

Frequency = 404\dfrac{40}{4} = 10 Hz

Time period = 1Frequency\dfrac{1}{\text{Frequency}}

Substituting we get,

Time period = 110\dfrac{1}{10} = 0.1 s

Hence, frequency = 10 Hz and time period = 0.1 s.

Chapter Overview: Sound

This chapter covers how sound is produced by vibrating objects, how it travels through media (solid > liquid > gas), and its characteristics: frequency (pitch), amplitude (loudness), and time period. The audible range, ultrasound applications, human ear anatomy, and noise pollution are important exam topics.

Board Exam Weightage: 5-7 marks | Difficulty: Moderate to High

Key Formulas & Concepts

ConceptDetails
Frequencyf = number of oscillations per second; Unit: Hertz (Hz)
Time PeriodT = 1/f; Time for one complete oscillation; Unit: seconds
Audible Range20 Hz to 20,000 Hz (for humans)
Sound SpeedSolid (~5000 m/s) > Liquid (~1500 m/s) > Gas (~340 m/s in air at 20°C)

Must-Know Concepts

  • Sound is produced by vibrating objects and needs a medium to travel (cannot travel in vacuum)
  • Loudness depends on amplitude; pitch depends on frequency
  • Men have deeper voices because their vocal cords are longer (lower frequency)
  • Ultrasound (>20,000 Hz): used in medical imaging (ultrasonography), SONAR, cleaning
  • Infrasound (<20 Hz): produced by earthquakes, elephants, whales
  • Noise pollution causes hearing loss, stress, high blood pressure; controlled by silencers, green belts, ear protection

Loudness vs Pitch

PropertyLoudnessPitch
Depends onAmplitudeFrequency
Greater → Louder soundHigher (shriller) sound
UnitDecibel (dB)Hertz (Hz)
ExampleShouting vs whisperingWoman's voice vs man's voice

Common Mistakes to Avoid

  • Confusing loudness (amplitude) with pitch (frequency)
  • Thinking sound can travel through vacuum
  • Errors in T = 1/f calculations (they are inversely related)
  • Mixing up ultrasound (>20,000 Hz) and infrasound (<20 Hz)

Scoring Tips

  • Practice T = 1/f problems: if f = 500 Hz, T = 0.002 s
  • Draw and label the human ear with all parts
  • Learn 3 applications each of ultrasound and noise prevention
  • Remember: solid > liquid > gas for speed of sound

Frequently Asked Questions

Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.

Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).

Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.