NCERT Class 11 Physics Solutions: Chapter 15 – Waves-Part 13

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Question 15.23:

A narrow sound pulse (for example, a short pip by a whistle) is sent across a medium. Does the pulse have a definite frequency, wavelength, speed of propagation?

If the pulse rate is after every , (that is the whistle is blown for a split of second after every ), is the frequency of the note produced by the whistle equal to .



Frequency: No

Wavelength: No

Speed of propagation: Yes


The narrow sound pulse does not have a fixed wavelength or frequency. However, the speed of the sound pulse remains the same, which is equal to the speed of sound in that medium.


The short pip produced after every does not mean that the frequency of the whistle is . It means that is the frequency of the repetition of the pip of the whistle.

Question 15.24:

One end of a long string of linear mass density is connected to an electrically driven tuning fork of frequency. The other end passes over a pulley and is tied to a pan containing a mass of the pulley end absorbs all the incoming energy so that reflected waves at this end have negligible amplitude. At the left end (fork end) of the string has zero transverse displacement and is moving along positive direction. The amplitude of the wave is . Write down the transverse displacement y as function of and that describes the wave on the string.


The equation of a travelling wave propagating along the positive direction is given by the displacement equation:

Linear mass density,

Frequency of the tuning fork,

Amplitude of the wave,

Mass of the pan,

Tension in the string,

The velocity of the transverse wave v, is given by the relation:

Angular frequency,


Substituting the values from equations in equation get the displacement equation: