wiz-icon
MyQuestionIcon
MyQuestionIcon
1
You visited us 1 times! Enjoying our articles? Unlock Full Access!
Question

A man standing on a platform observe that the frequency of the sound of a whistle emitted by a train drops by 140Hz. If the velocity of sound in air is 330m/s and the speed of the train is 70m/s, the frequency of the whistle is

A
571Hz
No worries! We‘ve got your back. Try BYJU‘S free classes today!
B
800Hz
Right on! Give the BNAT exam to get a 100% scholarship for BYJUS courses
C
400Hz
No worries! We‘ve got your back. Try BYJU‘S free classes today!
D
260Hz
No worries! We‘ve got your back. Try BYJU‘S free classes today!
Open in App
Solution

The correct option is B 800Hz
The question is based on Doppler effect. We can write a formula for apparent frequency (f) where a source ( train ) is moving away from the observation ( man ).
f=vfvfu
f= Apparent frequency
v= velocity of sound in air =330m/s
f= real frequency
u= velocity of source =70m/s
Now we can substitute above values into the formula
f=330f330+70

=330f400

=33f40(1)
But we know that,
real frequency apparent frequency =140Hz
ff=140
f33f40=140
7f40=140
f=140×407
Frequency of the whistle =800Hz.
Hence, the answer is 800Hz.

flag
Suggest Corrections
thumbs-up
1
Join BYJU'S Learning Program
similar_icon
Related Videos
thumbnail
lock
PHYSICS
Watch in App
Join BYJU'S Learning Program
CrossIcon