# A Car Moving at 108 Km H−1 Finds Another Car in Front It Going in the Same Direction at 72 Km H−1. the First Car Sounds a Horn that Has a Dominant Frequency - Physics

Sum

A car moving at 108 km h−1 finds another car in front it going in the same direction at 72 km h−1. The first car sounds a horn that has a dominant frequency of 800 Hz. What will be the apparent frequency heard by the driver in the front car? Speed of sound in air = 330 m s−1.

#### Solution

Given:
Velocity of car sounding a horn $v_s$= 108 km/h =$108 \times \frac{5}{18} \text { m/s }$= 30 m/s
Velocity of front car $v_0$= 72 kmh−1 =$72 \times \frac{5}{18} = 20 \text { m/s }$

Frequency of sound emitted by horn $f_0$= 800 Hz
Velocity of air v = 330 ms−1
Apparent frequency of sound heard by driver in the front car ($f$) is given by:

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

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

$f = \left( \frac{330 - 20}{330 - 30} \right) \times 800 = 826 . 67$

$\simeq 827 \text{ Hz }$

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 75 | Page 357