Question

Two trains A and B with speeds 20 m –1 and 30 m –1 respectively in the same direction on the straight track, with B ahead of A, the engines are at the front ends. The engine of train A blows a long whistle.

Assume that the sound of whistle is composed of components varying in frequency from f1 = 800 Hz to f2 = 1120 Hz, as shown in Fig. 9.8. The spread in the frequency (highest frequency – lowest frequency) is thus 320 Hz. The speed of sound in still air is 340 m *–1.

(i) The speed of sound of the whistle is

(A) 340 m –1 for passengers in A and 310 m –1 for passengers in B

(B) 360 m –1 for passengers in A and 310 m –1 for passengers in B

(C) 310 m –1 for passengers in A and 360 m –1 for passengers in B

(D) 340 m *–1 for passengers in both the trains.

(ii) The distribution of the sound intensity of the whistle as observed by the passengers in train A is best represented by

(iii) The spread of frequency as observed by the passengers in train B is

(A) 310 Hz (B) 330 Hz (C) 350 Hz (D) 290 Hz.

Answer 1

Correct Option (i) D (ii) A (iii) A

Explanation:

(i) The speed of sound does not depend on speed of source or observer.

(ii) Since there is no relative motion between the source and the observer, the frequency of sound heard by the passengers in train A will be same as the original frequency of sound emitted by the whistle of train A.

So the spread of frequency = 1085 – 775 = 310 Hz.