Question

a ringing mobile phone get slipped by hand of a boy standing at balcony at height 100m, the freq of sound heard by the person standing at balcony of height 55m from ground just before mobile phone passes him will be (actual frequency of ringing sound of mobile ia 200hz and velocity of sound is 330m/s)​

Answer 1

Given: A ringing mobile phone get slipped by hand of a boy standing at balcony at height 100m. Actual frequency of ringing sound of mobile is 200 Hz and velocity of sound is 330m/s.

To find: The frequency of sound heard by the person standing at balcony of height 55m from ground just before mobile phone passes him

Explanation: Let frequency of the sound be f, velocity of sound be v and velocity of mobile phone just before it passes the person be vs.

Distance that the mobile phone fell(d)

= 100-55= 45 m

Since the mobile phone starts from rest, initial velocity is 0.

Speed acquired can be given by:

$({vs)}^{2} = 2 \times g \times d$

$({vs)}^{2} = 2 \times 10 \times 45$

$({vs)}^{2} = 900$

vs= 30 m/s

When any of the source or observer is in motion, frequency changes. This apparent frequency can be given by Doppler's effect.

Here, the source that is the phone is in motion.

Let f1 be the apparent frequency.

f= 200 Hz

v= 330 m/s

The formula for Doppler's effect when the source travels towards the observer can be given by:

$f1 = ( \frac{v}{v - vs} ) \times f$

$f1 = (\frac{330}{330 - 30} ) \times 200$

$f1 = ( \frac{330}{300} ) \times 200$

f1 = 220 Hz

Therefore, the frequency heard by the person standing at the balcony just before mobile phone passes him will be 220 Hz.