A tutorial sheet of Doppler effect questions for sound is given below.
- The speed of sound in still air is v. A train blowing its whistle is moving to the east at a speed vs. What is the speed of sound at a point (a) east of the train, (b) north of the train, (c) west of the train, (d) south of the train.
- When a source of sound waves moves towards you do you measure an increase or decrease in the speed of the waves?
- When the moving source emitting sound waves is directly opposite the observer is there an observed frequency shift in the sound? [no]
- A train sounding its whistle moves to the east at a speed vs. An observer moves at a velocity vo towards the train. Is the observed frequency the same as the case when the observer is at rest and the train is approaching at a speed vs+vo?
- Draw the wave pattern when (a) vs < v, (b) vs = v and (c) vs > v. In the last case show the bow wave.
- A car traveling at 10 m/s sounds its horn, which has a frequency of 500 Hz, and this is heard in another car which is travelling behind the first car in the same direction at 20 m/s. The sound can also be heard in the second car by reflection from a bridge. If the speed of sound in air is 340 m/s what frequencies will the driver of the second car hear? [514 Hz,545 Hz]
- An observer in a mountain town hears a train whistle and 3.0 s later hears the start of the echo from a cliff. The echo's frequency is 0.90 that of the sound heard directly.(a) how far is the train from the cliff? (b) how fast and in what direction is the train moving? [510 m, 17.9 m/s towards observer, train is between cliff and the observer]
- A transmitter sends out waves of frequency f and speed v.A target moves towards the transmitter at a speed u. Show that the frequency of the reflected waves received back at the transmitter is f(v+u)/(v-u). If u is much smaller than v show that this expression becomes f(1+2u/v).