How Fast Was the NASCAR Race Car Going?

[TungYee32]

Homework Statement

George and Martha are at a NASCAR race and as the race car made the turn to their right and rushed towards them the frequency they heard was F0 = G5 or 821 Hz. However, after it passed the frequency dropped to F0/2. The direction the cars are traveling is the +x axis, what is the velocity of the race car?

Homework Equations

Wave velocity = frequency x wavelength

 

[mgb_phys]

Not the right equation - you are looking for the velocity of the car, not the sound.
Do you know about Doppler shifts?