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Homework Help: The observed frequency of a 1,200 Hz wave

  1. Aug 10, 2012 #1
    1. The problem statement, all variables and given/known data

    During a hurricane, a 1,200 Hz warning siren
    on the town hall sounds. The wind is blowing
    at 55 m/s in a direction from the siren toward
    a person 1 km away. With what frequency does
    the sound wave reach the person? (The speed of
    sound in air is 330 m/s.)

    (A) 1,000 Hz
    (B) 1,030 Hz
    (C) 1,200 Hz
    (D) 1,400 Hz
    (E) 1,440 Hz

    2. Relevant equations

    [tex]f = \left( \frac{c + v_r}{c + v_{s}} \right) f_0 \,[/tex]

    C is the velocity of waves in the medium;
    Vr is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source;
    Vs is the velocity of the source relative to the medium; positive if the source is moving away from the receiver.

    3. The attempt at a solution

    Stop me at any point if I'm wrong:

    C would be 330 m/s. Vr would be 55, and Vs would be 0?

    I try plugging this into the formula, and I always ~1030 Hz, but the answer key (this is from a practice GRE) says that the answer is 1200 Hz. How do they get this answer?
  2. jcsd
  3. Aug 10, 2012 #2
    Both the receiver and the source are moving at the same velocity with regard to the medium.
  4. Aug 10, 2012 #3
    Right right right, I just realized that before checking the response here, lol
  5. Aug 10, 2012 #4
    Okay, what about this one. I have no idea where to start. I know some of the equations for parallel plate capacitors, but none that have current in them.

    A large, parallel-plate capacitor consists of two
    square plates that measure 0.5 m on each side. A
    charging current of 9 A is applied to the capacitor.
    Which of the following gives the approximate rate
    of change of the electric field between the plates?
  6. Aug 10, 2012 #5
    Although I know that I = C dV/dt, but the capacitance of the system would rely on how far apart the plates are, which the question doesnt specify.
  7. Aug 10, 2012 #6
    Actually, C = eA/d, meaning that I = eA/d * dV/dt. The choices are all in V / (m.s) format, so I/eA = 1/d * dV/dt which would be dimensionally correct.
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