[2082] A train is approaching a cliff at 10 m/s. The driver sounds a whistle of frequency 600 Hz. What will be the frequency of echo as heard by the driver? (Velocity of sound in air = 340[latex]ms^{-1}[/latex]). [3] Ans: 636.36 Hz Solution: Here, Velocity of observer (vo) = 10 m/s Actual frequency of source (f) = 600 Hz Apparent frequency of echo as heard by car driver (f’) =? Velocity of sound in air = 340 m/s We have, The apparent frequency as heard by the driver is, f ‘ = [latex](\frac{v+v_o}{v-v_s})f[/latex]
[2081 GIE ‘A’] A car is sounding a horn of frequency 500 Hz and travelling towards a stationary observer with speed 20 m/s. What is apparent frequency heard by the observer? (Velocity of sound in air = 340 m/s). [3] Ans: 531.25 Hz Solution: Given, Frequency of source (f) = 500 Hz Velocity of source (vs) = 20 ms-1 Apparent frequency as heard by observer ([latex]f'[/latex]) =? Velocity of sound (v) = 340 m/s As source is moving towards the stationary observer, the apparent frequency as heard by the observer is given by: f’ = [latex]\frac{v}{v-v_s}\times f[/latex] = [latex]\frac{340}{340-20}\times 500[/latex] = 531.25 Hz
