Sound at 50 kHz can be used by police to measure a car's speed in a speed trap. If a car is travel at a speed relative to the source of the sound, a beat frequency will be produced when the sound returns to the source. The fast the car is moving, the larger the beat frequency produced. If the speed limit in a neighborhood is 40 mph, what is the largest beat frequency produced by a car still traveling at the speed limit? Assume that the speed trap is set up to bounce sound waves off the back of a car as it is driving directly away from the source of sound.