Unit: Waves
Year: 1994 Question#: 39
Question: Base your answers to questions 39 through 41 on the diagram below which shows a parked police car with a siren on top. The siren is producing a sound with a frequency of 680 Hz, which travels first through point A and then through point B, as shown. The speed of the sound is 340 meters per second.
39. If the sound waves are in phase at points A and B, the distance between the points
could be(1 ) 1
(2 ) 1/2
(3 ) 3/2
(4 ) 1/4
What is this question really asking?
Explanation by: Derek N
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Answer 1
1
CORRECT
Because the waves are in phase the distance (by definition) has to be 1
. If they were not in phase the answer would be anything other than a whole number
of waves.
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Answer 2
1/2
INCORRECT
Using the formula v=f
you would get 340 m/s = 680 Hz x 
. If done correctly you would get 
= (1/2) m. But that is not what is being asked.return to top
Answer 3
3/2
INCORRECT
This answer is a fraction meaning that the waves would not be in phase. return to top
Answer 4
1/4
INCORRECT
This answer is a fraction meaning that the waves would not be in phase. return to top
What's this question really asking?
What is the possible length between points A and B if points A and B are in phase? return to top
