Police Radio Detector
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Police blotter for July 2
Lost property: A woman told police an envelope with a deposit for the medical clinic she works at dropped out of her pocket as she was walking to a downtown bank. The envelope held $70 in cash and about $190 in checks.
Physics Wave Question - SIN Question?
A police cruiser sets up a novel radar speed trap, consisting of two transmitting antennas at the edge of a main north-south road. One antenna is 2.0 m [W] of the other. The antennas, essentially point sources of continuous radio waves, are fed from a common transmitter with a frequency of 3.0 x 10^9 Hz. The trap is set for cars travelling south. A motorist drives a car equipped with a radar detector along a east-west road that cross the main road at a level instersection 1.0 x 10^2 m [N] of the radar trap. The motorist hears a series of beeps upon driving west through the intersection. The time interval between successive quiet spots is 0.20s as the car crosses the main road. How fast is the car moving?
I assume that the beeps occur when the car passes through a maximum caused be the interference between the signals from the two antennas.
At this angle, sine and tangent are almost identical, to we can use the proportion
wavelength/distance between sources = distance between maxima/distance from antenna to the car
Wavelength = velocity/frequency = .1 m
0.1 m/ 2m = x/100 m
x = distance between maxima = 5 m
Since the car hits a maximum every .2 s
5m/0.20 s = 25 m/s