The six situations in the figure show pairs of cars, all of which are identical.

Question

The six situations in the figure show pairs of cars, all of which are identical. The velocity of each car is given in the figure. In each case, CAR #1 honks its horn. Rank the situations from highest to lowest based on the frequency heard by CAR #2. Ties are possible. Use the symbols > and = to indicate your ranking, listing any that are equal in alphabetical order. For example, if Y and Z are the same and are both greater than X, then you would enter Y=Z>X (note: Z=Y>X would be incorrect, since Y and Z, being equal, must be in alphabetical order). Rank from highest to lowest frequency.

Explanation / Answer

The Doppler effect (or shift) is the change in frequency of a wave for an observer moving relative to the source of the waves. Sound waves travel at a set speed. However, because the two cars have relative motion to each other, the perceived frequency change heard by Car 2 will be based on how fast the two cars change their relative motion to and from each other. F Car 2 is approaching Car 1 at 60 mph; Frequency extremely high B Car 2 is approaching Car 1 at 40 mph; Frequency very high D Car 2 is being approached by Car 1 by 20 mph; Frequency high E There is no relative motion; No frequency change A Car 2 is departing from Car 1 by 20 mph; Frequency low C Both cars are departing relative to each other by 40 mph; Frequency very low

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