Class 8 Science Chapter 13 |
Friction NCERT Solutions
1. Choose the correct answer.
Sound can travel through
(a) gases only (b) solids only
(c) liquids only (d) solids, liquids and gases.
Answer: (d) solids, liquids and gases
2. Voice of which of the following is likely to have minimum frequency?
(a) Baby girl (b) Baby boy
(c) A man (d) A woman
Answer: (c) A man
3. In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.
(a) Sound cannot travel in vacuum. (T/F)
(b) The number of oscillations per second of a vibrating object is called
its time period. (T/F)
(c) If the amplitude of vibration is large, sound is feeble. (T/F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
(f) Unwanted or unpleasant sound is termed as music. (T/F)
(g) Noise pollution may cause partial hearing impairment. (T/F)
Answer:
(a) Sound cannot travel in vacuum. (T)
(b) The number of oscillations per second of a vibrating object is called
its time period. (F)
(c) If the amplitude of vibration is large, sound is feeble. (F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T)
(e) The lower the frequency of vibration, the higher is the pitch. (F)
(f) Unwanted or unpleasant sound is termed as music. (F)
(g) Noise pollution may cause partial hearing impairment. (T)
4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called
__________.
(b) Loudness is determined by the __________ of vibration.
(c) The unit of frequency is __________
(d) Unwanted sound is called __________ .
(e) Shrillness of a sound is determined by the __________ of vibration.
Answer:
(a) Time taken by an object to complete one oscillation is called time period.
(b) Loudness is determined by the aptitude of vibration.
(c) The unit of frequency is Hertz (Hz).
(d) Unwanted sound is called noise .
(e) Shrillness of a sound is determined by the frequency of vibration.
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
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:
7. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak (b) Sitar (c) Flute
Answer:
(a) Dholak – Diaphragm (b) Sitar- String (c) Flute – Air Column
8. What is the difference between noise and music? Can music become noise sometimes?
Answer:
The unpleasant sounds around us are called noise. Noise is produced by the irregular vibrations of the sound producing source. For Ex: Running of mixer and grinder in the kitchen ,blowing of horns of motor vehicles ,bursting of crackers , barking of dogs , shattering of glass ,landing and flying of aeroplane ,sounds coming from construction site, all students start talking together loudly in a classroom.
The sounds which are pleasant to hear are called all music. They are produced by the regular vibrations of the sound producing source. All the musical instruments produce musical sounds. The speakers of radio, stereo system and television also produce musical sound. When a person sing a song he or she also produces musical sound.
If however a musical sound becomes too loud it becomes noise.
10. Explain in what way noise pollution is harmful to human.
Answer:
1) loud noise can cause great harm to our ears. Constant loud noise reduces the hearing power of our ears. loud noise can even damaged ears permanently and cause defness.
2)Loud noise can cause a person to lose concentration in his work or studies.
3) loud noise can cause an ailment called Hypertension.
4) loud noise can cause irritation and headache.
5) loud noise during night time disturbs our sleep. Continued lack of sleep is bad for health.
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:
The intensity of noise decreases as the distance between listener and source increases. I would suggest a house which is three lanes away from the roadside because it will away from noise pollution.
Noise pollution has many effects such as it cause irritation, headache, disturbs sleep, hypertension, lose of concentration, harms to ears etc.
12. Sketch larynx and explain its function in your own words.
Answer:
The human beings produce sound by using the voice box which is called larynx.The human voice box contains two ligaments known as vocal cords .They are attached to muscles which change the tension or stretching in the cords and the distance between the cords.
The muscle of vocal cords are completely relaxed due to which the vocal cords are separated and loose so that air from the lungs passes through them without producing any sound. When we are not talking or singing ,the two vocal cords are far apart with a lot of gap between them.When we want to speak ,the muscles of vocal cords contract due to which the two vocal cords become stretched and close together leaving only a narrow slit between them. The lungs pass a current of air between the two vocal cords. This air makes the vocal cords vibrate. And the vibrating vocal cords produce sound.When we talk or sing we actually make our vocal cords vibrate. And vibration of vocal cords by expelled air produce vocal sounds.
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 why?
Answer:
Speed of the light (300,000,000 m/s) is more than the speed of sound(330 m/s). Thus, lightning is seen first which is accompanied by thunder later.
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