A motorcyclist travel at cnstsant speed of 72km/h along a straight road toward an obesrver standing at the centre of bridge over the road . When the motor cyclist is distant, the obesrver hears a sound of 65hz from the engine of the motorcycle. What would be the frequency of sound heard by the obsrever after the motorcycle has passed under the bridge and away from obeserver? my ans is 57.5 hz. ( i managed to get the ans for this)
A second motorcyclist now rides alongside with part the first motorcyclist , at constsnt speed of 72km/h. The frequnecy f sound of 2nd motorsysle is lower than the first motorcycle. When 2 machines are treavelling towards the observer, from same direction , the intensity of sound varies with 3hz. What would be the frequency of variation of sound intensity after the motorcycles have passed the bridge and moving away from the obersver?
The Attempt at a Solution
f ( motorclcye 2 heard ny observer) = 65-3 =62hz
to find frequency of mototrcycle 2 , i have 62/((330)/(330-20)) = 58.2 hz.
apparent frequncy of motorcycle 2 moving away from obsrever : ((330)/(330+20)) x 58.2 = 54.9 hz.
my ans is 57.5hz-54.9hz= 2.6hz.
but the ans is 3.7 hz . why i am wrong?