Question

A stationary tuning fork is in resonance with an air column in a pipe. If the tuning fork is moved with a speed of 2 ms−1 in front of the open end of the pipe and parallel to it, the length of the pipe should be changed for the resonance to occur with the moving tuning fork. If the speed of sound in air is 320 ms−1, the smallest value of the percentage change required in the length of the pipe is

Answer 1

For open pipe resonance, f = nv / 4L

 

Let us consider for n = 1,

 

Initial length of pipe = L1 for f1

 

(f1=V/4L1)......(1)

 

Now due to Doppler’s effect,

 

f2=(320/320−2)f1

 

f2=(320/320−2)f1

 

∴

 

f2=v/4L2.....(3)

 

320/318×V/4L1=V/4L2

 

=> L1/L2=318/320

 

% change in length,

ΔL/L×100

(1−L2/L1)×100

(1−318/320)×100

200/320

= 0.62%

 

 

Percentage of change requires is 0.62%