6.1 A sound source is moving at constant velocity past a stationary observer - NSC Physical Sciences - Question 6 - 2017 - Paper 1

One practical application is ultrasound scanning, which uses the Doppler effect to measure the rate of blood flow.

To calculate the speed of the moving source, we can use the Doppler effect formula:

For the frequency approaching the observer:

f' = rac{v + v_s}{v} f

Substituting the known values:

$2600 = \frac{340 + v_s}{340} \times 2600$

For the frequency moving away from the observer:

$f' = \frac{v - v_s}{v} f$

Substituting the known values:

$1750 = \frac{340 - v_s}{340} \times 2600$

By solving these two equations, we find:

1. From the first equation:

$2600 imes 340 = 340 v + 2600 v_s$(1)

2. From the second equation:

$1750 imes 340 = 340 v - 1750 v_s$(2)

Subtract (1) from (2) to eliminate v, giving us:

$v_s = 66.44 ext{ m/s}$

(a) Moves towards the observer: The frequency will INCREASE as the distance decreases, leading to a higher perceived frequency.

(b) Moves away from the observer: The frequency will DECREASE as the observer experiences a lower frequency due to the increasing distance.

The term red shifted refers to the phenomenon where the spectral lines (light) from an object (like a star) are shifted towards longer wavelengths. This indicates that the object is moving away from the observer.

The frequency of the light observed from the star will DECREASE due to the red shift effect, indicating that the star is moving away from the observer.