Question

An ambulance travels down a highway at 120kmh^-1, its siren emitting sound of frequency 400Hz. What is the frequency heard by a passenger in a car traveling at 90kmh^-1 as the car approaches and

by-passes the ambulance? 

Answer 1

a) We can find the frequency from the Doppler Shift formula:

$$f_o=f_s(\dfrac{v+v_o}{v-v_s}),$$ $$f_o=400\ Hz\cdot(\dfrac{340\ \dfrac{m}{s}+25\ \dfrac{m}{s}}{340\ \dfrac{m}{s}-33.33\ \dfrac{m}{s}})=476\ Hz.$$

(b) We can find the frequency from the Doppler Shift formula:

$$f_o=f_s(\dfrac{v-v_o}{v+v_s}),$$ $$f_o=400\ Hz\cdot(\dfrac{340\ \dfrac{m}{s}-25\ \dfrac{m}{s}}{340\ \dfrac{m}{s}+33.33\ \dfrac{m}{s}})=337.5\ Hz.$$