A scientist is running at speed, u, toward a large, smooth wall while holding a tuning fork that vibrates at frequency, f. In terms of u, f, and v (the speed of sound), what is the frequency the scientist hears reflected from the wall, and what is the beat frequency she hears? Hint: there are two Doppler shifts to consider -- the wall is first an observer, and then it is a source reflecting at the frequency it "hears".