needing help with these old physics questions

Question

1. A man standing at a railway crossing notices that the frequency of a horn of an incoming train increases when the train is coming towards him. The actual frequency of the horn doesn't change 

A.The train must be traveling at constant speed

B.The speed of the train must be increasing as it approaches the man

C.The speed of the train must be decreasing as it approaches the man



2. A driver of a stationary car hears a siren of an approaching ambulance at a frequency of 280Hz. If the actual frequency of the siren is 240Hz, find the speed of the ambulance. Note that the speed of sound is 343m/s.

A.28 m/s

B.36m/s

C.49m/s

D.57m/s

Answer 1

The train must be increasing in speed. If it had a constant speed it would be a higher frequency but constant.

From the doppler equation for a moving source we have 240=250(343/(343-v)

so the speed of the ambulance is 14.29 m/s