Tina is driving her sports car down US1, traveling 27 m/s. She sees her friend Rita up ahead, waiting to cross the street at the next intersection. Tina quickly slams down on the horn. If the horn has a frequency of 400 Hz, which of the following is the only possible frequency heard by Rita, as Tina drives toward Rita? (2 points)

Explanation:

Tina speed Vs =27m/s

Rita speed Vo = 0m/s, she is not moving

Frequency from Tina horn is 400Hz

fs = 400Hz

Frequency Rita will hear as Tina moves closer ? fo

This is a situation whereby source is moving toward observer.

Using Dopler's effect of source moving towards observer

fo = fs [V/(V—Vs)]

Where V is speed of sound

Let assume V =343m/s

Vs is source speed = 27m/s

Fs is source frequency = 400

fo = 400[343 / (343-27)]

fo = 400 [ 343/316]

fo = 400 × 1.0854

fo = 434.2Hz

As expected, since the source is moving towards the observer, we expect the observe to hear high frequency.

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