Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11

# Two Trains Are Travelling Towards Each Other Both at a Speed of 90 Km H−1. If One of the Trains Sounds a Whistle at 500 Hz - Physics

Sum

Two trains are travelling towards each other both at a speed of 90 km h−1. If one of the trains sounds a whistle at 500 Hz, what will be the apparent frequency heard in the other train? Speed of sound in air = 350 m s−1.

#### Solution

Given:
Velocity of sound in air v = 350 ms−1
Velocity of source $v_s$ = 90 km/hour =$90 \times \frac{5}{18}$= 25 m/s
Velocity of observer $v_0$ = 25 m/s
Frequency of whistle $f_0$ = 500 Hz
Apparent frequency$\left( f \right)$ heard by the observer in train B is given by:

$f = \left( \frac{v - v_0}{v - v_s} \right) f_0$

On substituting the respective values in the above equation, we get:

$f = \left( \frac{350 + 25}{350 - 25} \right) \times 500 = 577 \text { Hz }$

The apparent frequency heard in the other train is 577 Hz.

Concept: Speed of Wave Motion
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#### APPEARS IN

HC Verma Class 11, 12 Concepts of Physics 1
Chapter 16 Sound Waves
Q 73 | Page 356