Doppler effect – JEE First

A collection of Doppler effect problems

April 9th, 2022jeefirst0 Comments

Here is a set of four problems on the Doppler shift of sound waves. The solutions to the problems are given at the end. If you need a refresher on the Doppler effect, the following note might be useful: How does Doppler effect work?

Question 1

While standing at a crosswalk, you hear a frequency of 560 Hz from an approaching police car. After the police car passes and is moving away from you, you hear a frequency of 480 Hz. What is the speed of the police car (take speed of sound to be $v=343$ m/s)?

13.1 m/s
21.1 m/s

…

How does Doppler effect work?

January 7th, 2022jeefirst0 Comments

The Doppler effect for sound is a perceived difference between the frequency of sound heard by an observer and the actual frequency emitted by a source, due to relative motion between the source and the observer. A familiar example of this phenomenon is the change in sound frequency of an ambulance or fire engine as it rushes past you. We give a visual explanation of this effect and derive the relation between the observed and emitted frequencies below.

First, recall that the velocity of sound in a medium, $v$ , is a property of the medium alone. That is, after the …

Doppler effect and beats

January 6th, 2022jeefirst0 Comments

JEE Advanced 2019 Paper 1, Question 15

A train S1, moving with a uniform velocity of $108$ km/h, approaches another train S2 standing on a platform. An observer O moves with a uniform velocity of $36$ km/h towards S2, as shown in figure. Both the trains are blowing whistles of same frequency $120$ Hz. When $\mathrm{O}$ is $600$ m away from S2 and distance between $\mathrm{S} 1$ and $\mathrm{S} 2$ is $800$ m, what is the number of beats heard by O?