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Let's go over the questions and solve them step by step.

### 8.1.1
**Question:**  
Name the phenomenon that explains the change in the observed frequency.

**Answer:**  
This is the **Doppler Effect**.

---

### 8.1.2
**Question:**  
Calculate the frequency of the sound that the observer hears while rushing towards the train.  
- Given:
  - \( f_s = 480 \, \text{Hz} \) (source frequency)
  - \( v = 340 \, \text{m/s} \) (speed of sound)
  - \( v_o = 1.45 \, \text{m/s} \) (observer’s speed towards the source)
  - The train is stationary, so \( v_s = 0 \, \text{m/s} \).

**Solution:**  
The **Doppler Effect formula** for a stationary source and a moving observer is:

\[
f' = f_s \times \left( \frac{v + v_o}{v} \right)
\]

Where:
- \( f' \) is the observed frequency,
- \( f_s \) is the source frequency,
- \( v \) is the speed of sound in air,
- \( v_o \) is the speed of the observer.

Now, substitute the values:

\[
f' = 480 \times \left( \frac{340 + 1.45}{340} \right)
\]

\[
f' = 480 \times \left( \frac{341.45}{340} \right)
\]

\[
f' = 480 \times 1.0043 \approx 482.06 \, \text{Hz}
\]

**Answer:**  
The observed frequency is approximately **482.06 Hz**.

---

### 8.1.3
**Question:**  
How will the observed frequency change if there is a heatwave and you experience temperatures of about 40°C? Will it **increase, decrease, or remain the same**?

**Answer:**  
The speed of sound increases with temperature. At higher temperatures (like 40°C), the speed of sound will be greater than 340 m/s. Since the train is stationary, a higher sound speed will reduce the relative effect of the Doppler shift, meaning the change in frequency will **decrease**.

**Answer:**  
The observed frequency will **decrease**.

---

### 8.2
**Question:**  
A stationary listener hears the sound of a passing ambulance. The graph shows how the frequency changes over time. Explain the trend seen in the graph.

**Explanation:**  
- The graph starts at a higher frequency (around 850 Hz), which means the ambulance is approaching the listener.
- As the ambulance passes by, the frequency drops sharply to around 680 Hz.
- After the ambulance moves away from the listener, the frequency remains at a lower value (around 680 Hz).

This pattern matches the **Doppler Effect**, where the frequency increases when the source approaches and decreases when the source moves away.

---

Let me know if you want further clarification or have any follow-up questions! Here are five related questions to deepen your understanding:

1. What would happen to the observed frequency if the train were also moving towards the observer?
2. How does the Doppler Effect apply to electromagnetic waves, such as light from stars?
3. Can the Doppler Effect be used to measure the speed of vehicles? If so, how?
4. What is the formula for the Doppler Effect when both the source and the observer are moving?
5. How does temperature affect the speed of sound, and what formula relates them?

**Tip:**  
In practical applications like weather forecasts or radar speed guns, the Doppler Effect plays a crucial role in measuring speeds and distances.

8.1 A train is waiting by the train station to depart. While rushing towards the train at a constant speed of 1.45 m/s, you hear the train’s whistle sounding its departure (The train’s whistle has a frequency of 480 Hz). As you approach the train, you can hear how the frequency changes. 

8.1.1 Name the phenomenon that explains the change in the observed frequency.
8.1.2 Calculate the frequency of the sound that the observer hears while rushing to the train. Take the speed of sound in the air to be 340 m/s.
8.1.3 How will the observed frequency change if there is a heatwave and you experience temperatures of about 40°C? Write only INCREASE, DECREASE or REMAINS THE SAME.
8.2 A stationary listener listens to the sound of a passing ambulance. The graph below shows the changes in the frequency of the sound of the siren detected by the listener.

Learn how to solve problems involving the Doppler Effect, such as calculating the observed frequency of a train's whistle as an observer moves toward the source. The problem also covers the effects of temperature changes on sound frequency.

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