NCERT Class 9 Science Solutions: Chapter 12 – Sound Part 5

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Question 6:

Q. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Answer:

The speed of sound is less than the speed of light sound of thunder takes more time to reach the Earth as compared to light. Hence, a flash is seen before we hear a thunder.

Question 7:

Q. A 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.

Answer:

Q 7 Image Showing Speed Formula.

Q 7 Image Showing Speed Formula

For a sound wave,

Speed wavelength Frequency

Given that the speed of sound in air

(i) For,

(ii) For,

Hence, for humans, the wavelength range for hearing is 0.0172 m to 17.2 m.

Question 8:

Q. Two 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.

Answer:

Let the length of the aluminium rod be .

Speed of sound wave in aluminium at

Therefore, time taken by the sound wave to reach the other end,

Speed of sound wave in aluminium at

Therefore, time taken by sound wave to reach the other end,

The ratio of time taken by the sound wave in air and aluminium:

Question 9:

The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Answer:

Q 9 Image Showing Frequency Formula.

Q 9 Image Showing Frequency Formula

Frequency is defined as the number of oscillations per second. It is given by the relation:

Number of oscillations Frequency Total time

Given, Frequency of sound

Total time

Number of oscillations/Vibrations

Hence, the source vibrates 6000 times in a minute, producing a frequency of 100 Hz.

Question 10:

Q. Does sound follow the same laws of reflection as light does? Explain.

Answer:

Sound follows the same laws of reflection as light does. The incident sound wave and the reflected sound wave make the same angle with the normal to the surface at the point of incidence. Also the incident sound wave, the reflected sound wave, and the normal to the point of incidence all lie in the same plane.

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