Question

A man on the platform is watching two trains, one leaving and the other entering the station with equal speed of $4 \mathrm{~m} / \mathrm{s}$. If they sound their whistles each of natural frequency $240 \mathrm{~Hz}$, the number of beats heard by the man (velocity of sound in air $=320 \mathrm{~m} / \mathrm{s})$ will be (A) 6 (B) 3 (C) 0 (D) 12

Name: Problem 28, Chapter 9: A Complete Resource Book in Physics for JEE MAIN
Uploaded: 2021-10-10T14:31:57-08:00
Duration: 2 min 10 s
Channel: Dheeraj Sharma
Description: A man on the platform is watching two trains, one leaving and the other entering the station with equal speed of 4 m / s. If they sound their whistles each of natural frequency 240 Hz, the number of beats heard by the man (velocity of sound in air =320 m / s) will be (A) 6 (B) 3 (C) 0 (D) 12

   A man on the platform is watching two trains, one leaving and the other entering the station with equal speed of $4 \mathrm{~m} / \mathrm{s}$. If they sound their whistles each of natural frequency $240 \mathrm{~Hz}$, the number of beats heard by the man (velocity of sound in air $=320 \mathrm{~m} / \mathrm{s})$ will be
(A) 6
(B) 3
(C) 0
(D) 12

A Complete Resource Book in Physics for JEE MAIN

Sanjeev Kumar 1st Edition

Chapter 9, Problem 28

Instant Answer

Solved by Expert Dheeraj Sharma

10/10/2021

Step 1

The Doppler effect formula is given by: $f' = f \cdot \frac{v + v_0}{v + v_s}$ where $f'$ is the observed frequency, $f$ is the source frequency, $v$ is the speed of sound, $v_0$ is the speed of the observer, and $v_s$ is the speed of the source. Show more…

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A man on the platform is watching two trains, one leaving and the other entering the station with equal speed of $4 \mathrm{~m} / \mathrm{s}$. If they sound their whistles each of natural frequency $240 \mathrm{~Hz}$, the number of beats heard by the man (velocity of sound in air $=320 \mathrm{~m} / \mathrm{s})$ will be (A) 6 (B) 3 (C) 0 (D) 12

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Key Concepts

Doppler Effect

The Doppler Effect is the phenomenon where the frequency of a wave changes due to the relative motion between the source and the observer. When the source of a sound moves towards a stationary observer, the observed frequency increases; when it moves away, the observed frequency decreases. This effect is essential for understanding how moving sources modify the pitch of sounds in real-world scenarios.

Beat Frequency

Beat Frequency arises from the interference of two sound waves with slightly different frequencies. When these waves are superimposed, they create fluctuations in sound intensity at a rate equal to the absolute difference between the two frequencies, which is perceived as beats. This concept is widely used in applications like tuning musical instruments and analyzing wave interference.

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