Lesson: Doppler Shift Physics • 9th Grade
In this lesson, we will learn how to explain the effect of the motion of the sources and the observers of waves on the measured frequencies of the waves.
Sample Question Videos
The graphs show the change in frequency with time of the sound of the horn of a fast-moving car that is heard by a person walking slowly by the side of the road as the car approaches and passes them. Which of the graphs (A), (B), (C), and (D) most correctly shows the changes in frequency that the pedestrian hears?
A firework is set up to launch horizontally in a fireworks display. As the firework’s fuse burns, it emits sound waves that have a frequency of 100 Hz and a wavelength of 3 m. When the rocket launches, it accelerates in a negligible time to a constant speed of 225 m/s and continues to emit sound waves with the same frequency and wavelength as before the launch, as shown in the diagram. An unwary person accidentally enters the firework display area and the firework travels directly toward them. The person is not moving.
According to the person, at what speed do wave fronts emitted from the rocket before it is launched travel?
According to the person, at what speed do wave fronts emitted from the flying rocket travel before the rocket reaches the point where the person stands?
The person manages to duck and avoid being hit by the firework, which then moves directly away from the person. According to the person, at what speed do wave fronts emitted from the rocket after it passes the person travel?
What is the frequency of the sound of the approaching firework as heard by the person?
What is the frequency of the sound of the receding firework as heard by the person?
Which of the following combinations of motion of a sound wave source and a sound wave observer produces the largest Doppler shift of the frequency of the wave as measured by the observer?