A train approaches a tunnel in a vertical cliff perpendicular to the rail road tracks.  It sounds its horn with a frequency of 250 Hz.  The speed of the train is 100 km/h and the speed of sound is 340m/s.

a. What is the frequency of sound heard by an observer standing at the entrance of the Tunnel?

b. What is the frequency of sound reflected by the cliff and heard by an engineer on the train?

2-7 The Doppler effect
Example 2