Joe, standing near a straight road, records the sound of the horn of a car trave
ID: 1782050 • Letter: J
Question
Joe, standing near a straight road, records the sound of the horn of a car traveling at a constant speed. The recorded frequency as a function of the car's position along the road for trials at different car speeds is accurately graphed below.
The speed in trial R is ..... to that in Q.
The distance of Joe from the path in R is ..... in Q.
The speed in trial U is ..... that in S.
When the car is far away, the average of the frequencies moving in and out is ..... the frequency at rest.
The speed in trial T is ..... that in other trials.
The speed in trial S is ..... that in other trials.
Choices are greater than, less than, or equal than
Using the data on the figure, calculate the speed of the car in trial S. Use 340 m/s for the speed of sound.
1050 1000 950 900 850 O 800 -100 -50 50 100 Position, (m)Explanation / Answer
Solution:
Doppler's effect gives the apparent frequency as a function of the speed of the observer and speed of the source of sound.
f ' = [(v+/- vo) /(v-/+vs) ]
When the car approaches the observer, the frequency increases. As the curve for R is almost straight, speed at R is GREATER than at Q .
The distance is LESS than in Q.
The speed in trial U is .LESS.... that in S. this is observed from the distance covered which is less at U
When the car is far away, the average of the frequencies moving in and out is Zero
The speed in trial T is LESS than that in other trials.
The speed in trial S is GREATER than that in other trials.
940 = 340 / (340 -vs) * 900
=> 1.04 = 340 /(340 -vs)
=> 340-vs = 340 /1.04
=> 340 - 325.5 =vs
=> vs = 15 m/s
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