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

1. Aug 10, 2012

### warfreak131

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

$$f = \left( \frac{c + v_r}{c + v_{s}} \right) f_0 \,$$

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. Aug 10, 2012

### voko

Both the receiver and the source are moving at the same velocity with regard to the medium.

3. Aug 10, 2012

### warfreak131

Right right right, I just realized that before checking the response here, lol

4. Aug 10, 2012

### warfreak131

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?

5. Aug 10, 2012

### warfreak131

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.

6. Aug 10, 2012

### warfreak131

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.