1. The problem statement, all variables and given/known data A source of sound emits waves at a frequency f 450 Hz. An observer is located at a distance d 150 m from the source. If the observer is moving away from the source at a velocity vobs 40 m/s, how does the number of wavefronts change with time? dN/dt ? (in Hz) 2. Relevant equations λ=u/f N=d/λ u - speed of sound 3. The attempt at a solution with moving observer my frequency changes. f'=(u/u+vobs)*fsource λ'=u/(u/u+vobs)*fsource)=(u+vobs)/f in time Δt, ΔN=(d+vobsΔt)/λ' - d/λ' then I find a limit for Δt>0 lim ΔN/Δt= dN/dt Am I on the right way?