- #1
CAF123
Gold Member
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I was considering the following question;
'A fire engine traveling at 70m/s drives past a van traveling at 30m/s. The speed of sound in air is 330m/s and the fire engine has a frequency of 200Hz when stationary. At what frequency does the van driver hear the siren?'
I understand you have to use f(prime) = f((v+/-vd)/(v+/-vs)), however I find it quite difficult to decide whether to use the + or - in the calculation, when things are moving relative to one another. An explanation of when to use a + and when to use a - in each of the numerator/denominator would be most helpful.
The answer is 180Hz.
Thanks.
'A fire engine traveling at 70m/s drives past a van traveling at 30m/s. The speed of sound in air is 330m/s and the fire engine has a frequency of 200Hz when stationary. At what frequency does the van driver hear the siren?'
I understand you have to use f(prime) = f((v+/-vd)/(v+/-vs)), however I find it quite difficult to decide whether to use the + or - in the calculation, when things are moving relative to one another. An explanation of when to use a + and when to use a - in each of the numerator/denominator would be most helpful.
The answer is 180Hz.
Thanks.