Question

A police officer with an exceptionally good ear hears an approaching motorcycle under full throttle making a 660 Hz tone and pulls the motorcycle over to give the driver a ticket. His radar gun lost the speed record but he notices the motorcycle's engine at maximum revolution makes a 560 Hz tone. (5 marks) a) Determine the speed of the motorcycle when the officer first heard it. The air temperature is 10 °C. (3 marks) b) What causes this frequency shift? Indicate with a diagram. Since this is a drawing question, feel free to put whateve you want in the blank, as we will be looking at your written work for the answer (3 marks)

Answer 1

Ans) (a) Observed frequency = f = 660 Hz oliginai faequency =.fo = 56012 speed of sound at oC = v= 33.7. 33 mm Doppler formula for approaching source gives IV-V V+ speed of the motorcycle. f Vs fo

Vsav Vs X337.33 V = fo/f 11- s = fo/f => ve (F fo/f Vs V=(1-5606 )x337 51.11061 mA (b) movement of the sonice alters the waveleng- th and hence the received frequency of the Sound Though the source frequency and ware velocity are unchanged. we can segard the sound as a register series of pulses separated by time to = 1/f folie frequency of the sound from the source: for a stationary source and the observe N's

in time I the sound travels a distance VT, and if the pulses are separated by a distance L (wave length), the number reaching the obser- is VsT/L: The rate at which the pulses assive is Ns/L, and this is the frequency of Sound to heard by the observer. for an approening source We have Ve to 'vto Ś During the interval to the first pulse travels a distance is to in the medium, and the source travels a distance to so this time L = Vs To V to

L = (Usv) To (us-v/fo f also 2 f= Vs fo Vs-V which is the same equation we used in is ) part (a)