In the following situations a light is being directed towards you by the object

Question

In the following situations a light is being directed towards you by the object described. In which of the following will there be a Doppler Effect? Choose all that apply.



A police car is circling you. You are stationary.

A fire truck is moving towards you while traveling 40 mph. You are stationary.

An ambulance is moving away from you while traveling 40 mph. You are stationary.

A train is stationary. You are stationary.

A flying saucer is moving away from you while traveling 40 ft/min. You are moving away from it while traveling 40 ft/min.

Explanation / Answer

doppler effect is the change in frequency of a wave for an observer moving relative to the source of the wave. In the first case, the velocity of the police car and you is the same, so there is no doppler effect. In the second case, there is doppler effect because there is a difference in the velocities of the 2 objects. In the third case, there is also a difference between the 2 velocities, so there is doppler effect. In the fourth case, because the source is circling around you, at any moment, the change in velocity is tangential and not in the direction the sound wave is travelling towards you, so there is no doppler effect. In the last example, there is a difference in the velocity again, so there is doppler effect.

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