Doppler Effect of a blown whistle

[lha08]

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

Homework Equations

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

 

[rl.bhat]

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.

 

[lha08]

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?

 

[rl.bhat]

Velocity v = 2*pi*n*r where n is the number revolution per second.

 

[lha08]

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]

but how do i solve for the radius using the length of the string?

Length of the string itself is the radius.