Class 8 Science


NCERT Solution

Question 1: Sound can travel through

  1. Gases only
  2. Solids only
  3. Liquids only
  4. Solids, liquids and gases

Answer: (d) solids, liquids and gases

Question 2: Which of the following voices is likely to have minimum frequency?

  1. Baby girl
  2. Baby boy
  3. A man
  4. A woman

Answer: (c) A man

Question 3: In the following statements, tick T against those which are true, and F against those which are false:

  1. Sound cannot travel in vacuum.
  2. The number of oscillations per second of a vibrating object is called its time period.
  3. If the amplitude of vibration is large, sound is feeble.
  4. For human ears, the audible range is 20 Hz to 20,000 Hz.
  5. The lower the frequency of vibration, the higher is the pitch.
  6. Unwanted or unpleasant sound is termed as music.
  7. Noise pollution may cause partial hearing impairment.

Answer: (a) T, (b) F, (c) F, (d) T, (e) F, (f) F, (g) T

Question 4: Fill in the blanks with suitable words.

  1. Time taken by an object to complete one oscillation is called ....................
  2. Loudness is determined by the .........................of vibration.
  3. The unit of frequency is..........................
  4. Unwanted sound is called ......................... .
  5. Shrillness of a sound is determined by the ................. of vibration.

Answer: (a) time period, (b) amplitude, (c) hertz, (d) noise, (e) frequency and amplitude

Question 5: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer: Given; number of oscillations = 40, time = 4 s
Time period can be calculated as follows:
Time taken for 40 oscillations = 4 second
Hence, time taken for 1 oscillation = 4 ÷ 40 = 0.1 second
Frequency can be calculated as follows:
Frequency = number of oscillations ÷ time
= 40 ÷ 4 = 10 Hz

Question 6: The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Answer: Since time period is time taken to complete one oscillation,
Time taken for 500 oscillations = 1 s
Hence, time taken for 1 oscillation = 1 ÷ 500 = 0.002 s

Question 7: Identify the part which vibrates to produce sound in the following instruments:

  1. Dholak
  2. Sitar
  3. Flute

Answer: (a) Stretched membrane, (b) String, (c) Air column

Question 8: What is the difference between noise and music? Can music become noise sometimes?

Answer: Unwanted sound that is unpleasant to ear is called noise. A pleasant sound is called music. Music can become noise at many instances. When someone tries to enjoy very loud music, it can be noise for someone else. When loud music is played during religious celebrations or marriages, it can be annoying for many people.

Question 9: List sources of noise pollution in your surroundings.

Answer: Running vehicles
Use of loudspeaker
Use of TV with high volume

Question 10: Explain in what way noise pollution is harmful to humans.

Answer: Noise pollution is harmful to humans in many ways. Constant exposure to noise pollution can create many health related problems; like insomnia, hypertension and may even lead to loss of hearing.

Question 11: Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Answer: I will suggest my parent to buy a house which is away from the roadside, because house at roadside would be noisy because of running vehicles. On the other hand, the house which is three lanes away from the roadside would be quieter.

Question 12: Sketch larynx and explain its function in your own words.

Answer: Larynx is composed of two stretched membranes; with some gap between them. When air passes through the larynx, the membranes or vocal cords vibrate and produce sound. That is why larynx is also known as sound box.

structure of larynx

Question 13: Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain?

Answer: We know that the speed of light much more than the speed of sound. Due to this, light reaches to us faster than sound. Hence, during lightning we see the streak of light earlier than hearing the sound of thunder.