Police radars determine speed by measuring the Doppler shift of radio waves reflected by a moving vehicle. The do so by determining the beat frequency between the reflected wave and the 10.5 GHz emitted wave. Some units can be calibrated by using a tuning fork; holding a vibrating fork in front of the unit causes the display to register a speed corresponding to the vibration frequency.A tuning fork is labbled "55 mph". What is the frequency of the tuning fork?