A train of length `l` is moving with a constant speed `upsilon` along a circular track of radius `R`. The engine of the train emits a sound of frequency `f`. Find the frequency heard by a guard at the rear end of the train.

Explanation

To find the frequency heard by a guard at the rear end of a train moving along a circular track, we can use the Doppler effect. Here’s a step-by-step solution:

### Step 1: Understand the scenario
The train is moving with a constant speed \( \upsilon \) along a circular track of radius \( R \). The engine emits a sound of frequency \( f \). The guard is located at the rear end of the train.

### Step 2: Identify the source and observer
In this case:
- The source of the sound is the engine of the train, emitting sound with frequency \( f \).
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