Frequency of these reflected waves?

Question

a stationary motion detector sends sound waves of frequenct 0.150 MHz toward a truck approaching at a speed of 45.0 m/s. What is the frequency of the waves reflected back to the detector?

Explain your work. Thanks! :)

Answer 1

The waves are transmitted at a rate f = C/λ. They are encountered and reflected at a rate = (C+V)/λ. Thus the detector receives a frequency greater than the transmitted frequency by a factor of (C+V)/C = (340+45)/340. Multiply by 0.15 MHz and that is the received frequency.