Physics - Sound Wave Question with Solution | TestHub

By Doppler's Effect When observer is moving with velocity $v_0$ towards a source at rest then approach frequency $$N_{\text{Approach }}=N\left(\frac{v+v_0}{v}\right)$$ where $N=850\mathrm{Hz},v=340{\mathrm{ms}}^{-1},v_0=72{\mathrm{kmh}}^{-1}$ $$=20{\mathrm{ms}}^{-1}=850\left(\frac{340+20}{340}\right)$$ Similarly. when observer is moving away from source, then $$N_{\text{Soparation }}=N\left(\frac{v-v_0}{v}\right)=850\left(\frac{340-20}{340}\right)$$ The different of the two frequency. $$\begin{array}{l}{N}_{\text{Approuch }}-N_{\text{Soparation }}\\ \begin{array}{rl}& =850\left(\frac{360}{340}\right)-850\left(\frac{320}{340}\right)\\ & =\frac{850}{340}\times 40=\frac{850}{8.5}=100\mathrm{Hz}\end{array}\end{array}$$