A train, standing in a station yard, blows a whistle of frequency 400Hz in still air. The wind starts blowing in the direction from the yard to the station with a speed of 10m//s. Given that the speed sound in still air is `34om//s,

Options

A. the frequency of sound as heard by an observer standing on the platform is 400 Hz
B. the speed of sound for the observed standing on the platform is 350 m/s
C. the frequency of sound as heard by the oberver standing on the platform will increases
D. the fequency of sound as heard by the observer standing on the platform will decreases

Explanation

To solve the problem step by step, we will analyze the situation involving the train whistle, the observer, and the effect of the wind on the sound frequency.

### Step 1: Identify the given information
- Frequency of the whistle (f) = 400 Hz
- Speed of sound in still air (V) = 340 m/s
- Speed of wind (V_wind) = 10 m/s (blowing from the yard to the station)

### Step 2: Determine the effective speed of sound
Since the wind is blowing in the same direction as the sound, the effective speed of sound (V_effective) for the observer will be the speed of sound in still air plus the speed of the wind:
\[ V_{\text{effective}} = V + V_{\text{wind}} \]
\[ V_{\text{effective}} = 340 \, \text{m/s} + 10 \, \text{m/s} = 350 \, \text{m/s} \]

### Step 3: Analyze the frequency heard by the observer
The frequency of sound heard by the observer (f_apparent) can be calculated using the formula for the Doppler effect when both the source and observer are stationary:
\[ f_{\text{apparent}} = f \cdot \frac{V_{\text{effective}}}{V} \]
Since the observer is stationary and the source is also stationary, we can simplify this to:
\[ f_{\text{apparent}} = f \]
Thus, the frequency of sound as heard by the observer remains:
\[ f_{\text{apparent}} = 400 \, \text{Hz} \]

### Step 4: Conclusion on frequency and speed of sound
- The frequency of sound as heard by the observer is **400 Hz** (no change).
- The effective speed of sound for the observer is **350 m/s**.

### Step 5: Evaluate the options
- **Option A**: The frequency of sound as heard by an observer standing on the platform is 400 Hz. **(Correct)**
- **Option B**: The speed of sound for the observer standing on the platform is 350 m/s. **(Correct)**
- **Option C**: The frequency of sound as heard by the observer standing on the platform will increase. **(Incorrect)**
- **Option D**: The frequency of sound as heard by the observer standing on the platform will decrease. **(Incorrect)**

### Final Answer
- The correct options are A and B.

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