Chapter 17, Problem 17.73CP

Equation 16.40 states that at distance r away from a point source with power (Power)_avg, the wave intensity is

$I=\frac{{(Power)}_{\text{avg}}}{4\pi r^2}$

Study Figure 16.25 and prove that at distance r straight in front of a point source with power (Power)_avg moving with constant speed v_S the wave intensity is

$I=\frac{{(Power)}_{\text{avg}}}{4\pi r^2}\left(\frac{v-v_S}{v}\right)$