A learner investigates the relationship between the observed frequency and the frequency of sound waves emitted by a stationary source - English General - NSC Physical Sciences - Question 6 - 2022 - Paper 1

One application in the medical field of the Doppler Effect is the measurement of fetal heartbeat using a Doppler fetal monitor.

The type of proportionality that exists between f l and f s is directly proportional. This indicates that as the source frequency (f s ) increases, the observed frequency (f l ) also increases linearly.

To calculate the velocity of the learner approaching the sound source, we can use the relationship defined by the Doppler Effect formula:

$f_l = \frac{v + v_l}{v} f_s$

Given the gradient from the graph is 1.06, we can rearrange this to:

$1.06 = \frac{(340 + v_l)}{340}$

Substituting the known values and solving for the learner's velocity (v_l):

1.06 = \frac{(340 + v_l)}{340}

Multiplying both sides by 340:

340 * 1.06 = 340 + v_l

This gives:

v_l = 340 * 1.06 - 340

v_l = 20.4 , m/s

Thus, the magnitude of the velocity at which the learner approaches the sound source is 20.4 m/s.

In your answer book, draw the original graph labeled A, which represents the relationship between f l and f s for the given velocity. Then, for the graph representing the learner moving at a HIGHER velocity, sketch a second line labeled B, which should be steeper than graph A. This illustrates that at a higher velocity, the observed frequency increases more rapidly with an increase in the source frequency.