A lidar (laser radar) gun is an alternative to the standard radar gun that uses

Question

A lidar (laser radar) gun is an alternative to the standard radar gun that uses the Doppler effect to catch speeders. A lidar gun uses an infrared laser and emits a precisely timed series of pulses of infrared electromagnetic waves. The time for each pulse to travel to the speeding vehicle and return to the gun is measured. In one situation a lidar gun is a stationary police car observes a difference of 1.30 times 10^-7 s in round-trip travel times for two pulses that are emitted 0.450 s apart. Assuming that the speeding vehicle is approaching the police car essentially head-on, determine the speed of the vehicle.

Explanation / Answer

in 0.45 s the car travels

2d = c delta t

d=1/2* 3x10^8 *1.30x10^-7 = 60*1.3 = 19.5 m

the speed is then

19.5/.45 = 43.33 m/s

or 96.95 mph

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