# Why is it not option A? (Doppler effect)

#1
I used the equation change in f/f = v(train)/v(sound)

Image below
https://imgur.com/a/bEmCVeO
Last edited by ae86_trueno; 4 weeks ago
0
4 weeks ago
#2
Okay so firstly use the formula Velocity = lambda * frequency

This is some what of a ratio proportion type concept.

speed= 340 ms-1
so 340/0.85f = wavelength
= 400

If f was 1.00 then the wavelength would have been 340 so the change in wavelength is 60 meters
Now speed = distance/ time
Distance is 60 meters time take is 1 second
so speed is 60 ms-1

Now if you notice the frequency has reduced from 1 to 0.85 so when frequency reduces and wavelength increase this would mean that the distance between the lady and the sound source has increased. Therefore the source of sound is moving away from her.
0
4 weeks ago
#3
The change frequency is f - 0.85f and the denominator should be 0.85f when you put this in a fraction and factor our f you are given f(1 - 0.85) / 0.85f. The f cancel out meaning you have 0.15/0.85 x 340ms^-1 which gives you 60ms^-1
0
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