# Doppler Effect of a blown whistle

## Homework Statement

A whistle of frequency 500 Hz tied to the end of a string of length 1.2 m revolves at 400 rev/min. A listener standing some distance away in the plane of rotation hears frequencies in the range ____?
(speed of sound=340 m/s)
a) 436 to 586
b) 426 to 574
c) 426 to 584
d) 436 to 674

## The Attempt at a Solution

I'm not really sure how to approach this particular problem...i'm lost especially for the units and what to do with the string...thanks

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rl.bhat
Homework Helper
when the whistle is moving in a horizontal circle, at two points on the circle the whistle is moving either towards the observer or away from him. Apply Doppler effect to find the frequencies.

when the whistle is moving in a horizontal circle, at two points on the circle the whistle is moving either towards the observer or away from him. Apply Doppler effect to find the frequencies.
okay i understand that part, but how do i convert 400 rev/min and for the length of the string do i use C=2(pi)radius and solve for r?

Last edited:
rl.bhat
Homework Helper
Velocity v = 2*pi*n*r where n is the number revolution per second.

Velocity v = 2*pi*n*r where n is the number revolution per second.
but how do i solve for the radius using the length of the string?

rl.bhat
Homework Helper
but how do i solve for the radius using the length of the string?
Length of the string itself is the radius.