A car is moving to the left at 20 m/s while a truck is movingto the right at 25

Question

A car is moving to the left at 20 m/s while a truck is movingto the right at 25 m/s. If the truck emits a sound of frequency5000 Hz, what is the perceived frequency of the sound by the driverof the car? Assume the vehicles are moving directly away from eachother on a 20.0° C day. A)4384 Hz B)4706 Hz C)4932 Hz D)5079 Hz E)5714 Hz A car is moving to the left at 20 m/s while a truck is movingto the right at 25 m/s. If the truck emits a sound of frequency5000 Hz, what is the perceived frequency of the sound by the driverof the car? Assume the vehicles are moving directly away from eachother on a 20.0° C day. A)4384 Hz B)4706 Hz C)4932 Hz D)5079 Hz E)5714 Hz

Explanation / Answer

Let the perceived frequency of the sound by the driver ofthe car be fL which is given by fL = (v + vL/v + vS) *fS ----------(1) where v is the velocity of sound in air,vL is thevelocity of the listener,vS is the velocity of thesource and fS is the frequency of the source. The speed of sound in air is v = 340 m/s The speed of the car is vL = -20 m/s The speed of the truck is vS = +25 m/s The frequency of sound emitted by the truck is fS =5000 Hz The perceived frequency of the sound by the driver of the caris fL. Substituting the above values in equation (1) we get theperceived frequency of the sound by the driver of the car that isfL. where v is the velocity of sound in air,vL is thevelocity of the listener,vS is the velocity of thesource and fS is the frequency of the source. The speed of sound in air is v = 340 m/s The speed of the car is vL = -20 m/s The speed of the truck is vS = +25 m/s The frequency of sound emitted by the truck is fS =5000 Hz The perceived frequency of the sound by the driver of the caris fL. Substituting the above values in equation (1) we get theperceived frequency of the sound by the driver of the car that isfL.

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