Question

A train is moving on a straight track with speed $20\, ms^{-1}.$ It is blowing its whistle at the frequency of $1000\, Hz$. The percentage change in the frequency heard by a person standing near the track as the train passes him is close to (speed of sound $= 320\,ms^{-1}$)

• A. $12 \%$
• B. $6 \%$
• C. $18 \%$
• D. $24 \%$

Answer 1

Answer: (A)

$12 \%$

Answer 2

$f _{1}=1000\left(\frac{320}{300-20}\right)=1066\, Hz$
$f _{2}=1000\left(\frac{320}{300+20}\right)=941 \,Hz$
$\therefore$ Change is $\simeq 12 \%$