Today we talked about the Doppler Effect. how an observer can observe a higher or lower frequency than a source is actually emitting. This effect can happen if the observer and/or the source are moving towards/away from each other.

We derived the equation that governs how the observed frequency (fo) relates to the regular frequency of the source (fs). That equation is (fo)=(fs)*[v +/- vo]/[v +/- vs]. Your job in this essay is to explain how you know to choose the plus or the minus in the numerator and denominator. Pick at least two sample cases and for each sample case include both a physical reason you'd expect fo to go up or down (think about how sound waves are squishing together or spreading apart) and a mathematical reason (like will picking a + in the denominator increase or decrease fo?)