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Homework Help: Doppler Effect Question

  1. Mar 28, 2006 #1
    Hey guys,
    Im having lots of problems with this question..

    A car moving at 40m/s and a truck moving at 15m/s travel along the same straight road. The car's horn has a natural frequency of 400Hz. What is the change in the frequency observed by the truck as the car passes the truck? Assume the car and the truck are traveling ( a) in the same direction and ( b) in opposite directions

    Im using the car as the source, and the truck as the observer. The speed of sound is 340 m/s.

    The solutions I got are:

    a) [(340 + 15)/(340 - 40)] 400
    b) [(340 - 15)/(340 + 40)] 400

    according to the teachers answers, they are wrong..

    Thanks in advance :)
  2. jcsd
  3. Mar 28, 2006 #2


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    The question seems to be to claculate the change of frequency as the car passes the truck.

    when they are traveling in the same direction, calculate the frequency when the car is behind the truck, then calculate the frequency when the car is ahead of the truck. Subtract the two (the first will be larger, of course).

    For b) calculate the frequency heard when the car ia in front of the truck (and coming toward it) and then when th ecar is behind (and moving away). Subtract the two.

    If I understood the question correctly, thsi should give you the correct answers.

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