CBSE-IX-Science
10: 2-Sound
- #Science Sound part 2
- Qstn #1What is sound and how is it produced?Ans : Sound is a form of energy that produces a sensation of hearing in our ears.
Sound gets produced when any object vibrates/oscillates.
- Qstn #2Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.Ans :
- Qstn #3Cite an experiment to show that sound needs a material medium for its propagation.Ans : Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump If you press the switch you will be able to hear the bell. Now start the vacuum pump. When the air in the jar is pumped out gradually, the sound becomes fainter, although the same current is passing through the bell.
After some time when less air is left inside the bell jar you will hear a very feeble sound. Now if we evacuate the bell jar no sound is heard.
Result: The above mentioned activity shows that sound needs a medium to propagate.
- Qstn #4Why is sound wave called a longitudinal wave?Ans : Sound wave is called a longitudinal wave because sound waves travel in the air through compressions and rarefactions.
- Qstn #5Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?Ans : Pitch of the sound wave.
- Qstn #6Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?Ans : Since speed of thunder (sound) is much less (332 m/s) as compared to speed of flash (light) which is about
therefore light travels faster than sound hence thunder is heard a few seconds after the flash is seen.
- Qstn #7A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as
.Ans : For 20 Hz sound waves the wavelength would be
= v/n = 344/20 = 17.2 m
For 20 kHz sound waves the wavelength would be
/20000 Hz = 0.0172 m
- Qstn #8Two 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 air and in aluminium to reach the second child.Ans : Since speed of sound in air = 344 m/s
and speed of sound in aluminium = 6420 m/s
we know that v = distance/time therefore time = d/v
time taken by sound wave in air/time taken by sound wave in aluminium
= d/344: d/6420 = 6420/344 = 18.66/1
the sound will take 18.66 times more time through air than in aluminium in reaching other boy.
- Qstn #9The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?Ans : Frequency of source of sound being 100 Hz means the sound source vibrates 100 times in one second.
therefore vibrations made by sound source in 1 min (60 sec) =
= 6000
- Qstn #10Does sound follow the same laws of reflection as light does? Explain.Ans : Yes. Sound follows the same laws of reflection as light does. We can say that because here the directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane.
- Qstn #11When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?Ans : As the sensation of sound persists in our brain for about 0.1 s.To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. There for the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least
= 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m. Speed of sound will increase with increase in temperature. Therefore, on a hotter day speed of sound will be greater hence echoes may be heard more than once because of multiple reflections of sound result will be no distinct echo will be heard by us.
- Qstn #12Give two practical applications of reflection of sound waves.Ans : Two practical applications of reflection of sound waves
i. Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions.
ii. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.
- Qstn #13A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given,
and speed of sound =
.Ans :
= 10000
v =
= 100 m/s
we also know that v = u + gt = 0 + 10t
100 = 10t or, Time taken by stone to reach the pond surface (t) = 100/10 = 10 sec
therefore, time taken by sound to reach the top from pond surface = d/v = 500/340
= 1.47 sec
so the total time taken for splash being heard at the top = 10 + 1.47 = 11.47 s
- Qstn #14A sound waves travels at a speed of
. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?Ans : Since we know that
339 = 0.015
ν = 339/0.015 = 22600 Hz
Since the resulting frequency is beyond the audible range of human beings (20Hz to 20kHz) therefore sound will not be audible to human ears.