3. Give scientific reasons.

(a) In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.

Ans: 

(i) While the train runs on the rail, its wheels bang on the rail and friction occurs.

(ii) This banging and friction produce vibrations, and sound is produced.

(iii) These vibrations propagate over a large distance through a solid medium such as a rail. That means the sound travels over a large distance through the rail.

(iv) If an ear is put to the rail, this sound can be clearly heard and the arrival of a train can be guessed.

Thus, in earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.

(b) The sounds generated by a tabla and a sitar are different.

Ans:

(i) The stretched diaphragm of a tabla, when struck, produces vibrations that generate the sound.

(ii) The stretched string of a sitar, when plucked, produces vibrations that generate the sound.

(iii) In this way, due to the difference in the vibrating object, the frequency of vibrations from both objects is different, and the pitch of sound is also different. 

Thus, the sounds generated by a tabla and a sitar are different.

(c) If you were both on the moon, your friend will not be able to hear you call.

Ans:

(i) A medium is necessary for the propagation of sound.

(ii) There is no atmosphere on the moon as there is on earth, and thus, due to the absence of a medium, sound does not propagate while talking on the moon.

This is the reason why your friend will not be able to hear your call.

(d) We can hear the movement of a mosquito’s wings but we cannot hear the movement of our hands.

Ans:

(i) The up-and-down movement of a mosquito’s wings occurs very quickly. In one second, its wings move about 300 to 600 times. That means the frequency of movement of the wings is 300 Hz to 600 Hz. Thus, audible sound is produced, and the movement of the wings can be heard.

(ii) On the other hand, we cannot move our hands so rapidly. The frequency of sound produced by the movement of hands is less than 20 Hz. We cannot hear this infrasonic sound. Thus, the movement of our hands cannot be heard.

4. Write answers to the following questions.

(a) How is sound produced ?

Ans: Vibrations are produced in an object when it is struck. A vibrating object is a source of sound. The rhythmic vibrations of an object produce sound.

(b) What does the intensity of sound depend upon?

Ans:

The intensity of sound depends upon:

(i) Amplitude of vibrations: The intensity of sound is proportional to the square of the amplitude of vibration.

(ii) Frequency of fibrations: If frequency is higher, then intensity of sound is also higher.

(iii) Distance from the source of sound: If the distance between the listener and the source of sound is less, the intensity of the sound can be felt to be greater.

(c) Explain how the frequency of oscillation is related to the length of a pendulum and the amplitude of its oscillation.

Ans:

(i) As the length of a pendulum increases, the frequency of oscillations decreases. That means that as the length of a pendulum increases, the number of oscillations in one second decreases.

(ii) The frequency of oscillations is not affected much by the amplitude of oscillations. That means that even if the amplitude is increased, the frequency does not change much; it remains nearly the same.

(d) Explain the two ways by which the pitch of the sound generated by a stretched string can be changed.

Ans:

(i) When the tension in the wire is increased, the frequency increases, and thus the sound produced is louder. Also, when the tension in the wire is decreased, the frequency decreases, and the sound produced is less shrill.

(ii) When the length of the wire is decreased, the frequency increases. Thus, the sound produced is sharper. When the length of the wire is increased, the frequency decreases, and the sound produced is less shrill. Using these two ways, the pitch of the sound generated by a stretched wire can be changed.