Choose the scenario in which the sound frequency of the waves is higher.
A) the sound source moves towards you at 50 m/sec
Eliminate
B) the sound source moves towards you at 100 m/sec
C) the sound source moves away from you at 50 m/sec
D) the sound source moves away from you at 100 m/sec

B) the sound source moves towards you at 100 m/sec

Explanation:

The Dopper Effect is a phenomenon that occur when there is relative motion between an observer and a source of a wave. When this situation occurs, there is an apparent shift in frequency of the wave, as observed by the observer.

The apparent frequency observed by the observer is given by

where

f is the original frequency of the wave

f' is the apparent frequency

v is the speed of the wave

is the velocity of the observer (positive if moving towards the source of the wave, negative otherwise)

is the velocity of the source (negative if moving towards from the observer, positive otherwise)

In this problem, we want to find the scenario in which the sound frequency is higher.

We see that in all 4 scenarios, the sound source is moving: this means we have to find the scenario in which the denominator of the equation is smaller.

First of all, we notice the sound source moves towards the observer, is negative, so the denominator is higher: this means that the correct option must be either A or B.

Also, we notice that since is negative, a value larger in magnitude will mean a smaller denominator: therefore, the correct answer will be

B) the sound source moves towards you at 100 m/sec

Since this situation will make the denominator of the formula the smallest possible.

this would be about 43%