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Doppler Effect & Study Guide 1.
2.
A police car with a 600 Hz siren is traveling along the same street as a motorcycle. The velocities of the two vehicles and the distance between them are given in each figure. Rank from highest to lowest frequency of the siren as measured by the motorcycle rider. Highest f: B > A > D > C Lowest F A train approaches a station at a constant speed, sounding its whistle continuously. Three students are discussing what an observer standing at a station would hear as the train is approaching: Anish: “The train is not accelerating or decelerating. I think the observer will hear a constant pitch that matches the pitch of the whistle.” Brooke: “ Even if it isn’t accelerating, the observer will hear a higher pitch than the whistle actually emits since the train is moving toward the observer. I agree that the observer will hear a constant pitch.” Cruz: “I agree with Brooke that he observer will hear a higher pitch, but I think the observer will also hear the frequency increase constantly as the train gets closer and closer.” Which, if any, if these students do you agree with? Why? I agree with Brooke. The observer will hear a constant pitch because the train is not accelerating. The pitch the observer hears will be higher pitched than the stationary whistle emits (or higher than hear if in the same frame as the whistle, more on “frames” next unit ☺ ). The reason the frequency is higher is because the train is coming TOWARDS the observer. 3. What is the Doppler Effect? Give one practical application of this phenomenon. The Doppler Effect is when the frequency of a sound changes based on relative movement of the source of the sound and the observer of the sound. Some practice applications are: police radar guns (use to measure your speed and potentially give you a ticket), Doppler Radar used in measuring the direction and velocity of clouds when predicting the weather, Doppler shifts in light are used in astronomy to find exoplanets, or see #11… 4. In which scenario would a physics student hear the highest pitched car horn: (1) If they were in the car honking the horn, (2) If they were stationary on the side of the road while a car drove towards them honking the horn, (3) If there were two cars driving towards each other and the student was in one car and the other car was honking its horn, (4) If there were two cars driving away from each other and the student was in one car and the other car was honking its horn. 3 is the highest pitch. Ranking from greatest frequency to lowest frequency would be: 3, 2, 1, 4 5. You are standing at x = 0 m, listening to a sound that is emitted from a source at a frequency of 340. Hz. At t = 0 sec, the sound source is at x = 20 m and moving toward you at a steady 10 m/s. Sketch a graph (graph: f = y-‐axis; t = x-‐axis) showing the frequency you hear from t = 0 sec to t = 4 sec. 6. A train with a 1,000 Hz whistle passes by a train station at a constant speed of 30 m/s. A speaker on the station platform emits a 700 Hz siren as the train approaches. a. As the train approaches the station, is the frequency of the train whistle as perceived by an observer on the station platform (i) greater than 1,000 Hz, (ii) less than 1,000 Hz, or (iii) equal to 1,000 Hz? Explain your reasoning. i) greater than 1,000 Hz because the source is moving towards the observer. b. As the train approaches the station, is the frequency of the station warning siren as perceived by a passenger on the train (i) greater than 700 Hz, (ii) less than 700 Hz, or (iii) equal to 700 Hz? Explain your reasoning. i) greater than 700 Hz. The observer is moving towards the source. c. As the train moves away from the station, is the frequency of the train whistle as perceived by an observer on the station platform (i) greater than 1,000 Hz, (ii) less than 1,000 Hz, or (iii) equal to 1,000 Hz? Explain your reasoning. ii) less than 1,000 Hz because the source is moving away from the observer. d. As the train moves away from the station, is the frequency of the station warning siren as perceived by a passenger on the train (i) greater than 1,000 Hz, (ii) less than 1,000 Hz, or (iii) equal to 1,000 Hz? Explain your reasoning. ii) less than 1000 Hz. The observer is moving away from the source. 7. A wave travels to the right what is the motion like of the particles that make up the wave if the wave is (1) longitudinal? (2) transverse? Right AND LEFT Up and down 8. What causes beats? Give one practical application of this phenomenon. Beats are caused when sound of different frequencies interfere with each other. An application of beats would be tuning your instrument. When the instrument’s frequency matches the desired frequency, when played together (desired and instrument) one would hear no beats. 9. Suppose you set up a standing wave on a rope with three segments. If you shake with twice the frequency, how many wave segments will occur in your new standing wave? Double the frequency " half the wavelength, if speed stays the same. 3 segments (1.5 waves) " 6 segments (3 waves) 10. Light travels at the “speed of light” (3.0x108m/s). If red light has a longer wavelength than violet light, which has a greater frequency? Violet light would have a shorter wavelength and therefore a higher frequency (if speed is the same, which it is). 11. Astronomers find that light coming from point A at the edge of the sun has a slightly higher frequency than light from point B at the opposite side. What do these measurements tell us about the sun’s motion? Point A is moving towards us while point B is moving away from us. This points towards the run rotating on its axis. 12. Why do different objects have different sounds when dropped to the floor? Different objects have different resonant frequencies (different shapes, made of different materials, ect). Therefore when they are hit, such as dropping on the floor, they ring with different fundamental frequencies. 13. What TWO physics mistakes occur in a sci-‐fi motion when you see and hear at the same time a distant explosion in outer space? 1) Light travels faster than sound. You would see the explosion WAY before you hear it. (If you are standing near the finish line at a track meet during the 200 m dash, you can see the starting gun go off all the way across the field before you can hear the gun go off.) 2) Sound is a mechanical wave. It needs a medium to travel through. It cannot travel through the vacuum of space. 14. A pipe open at both ends has a fundamental frequency of 300. Hz when the temperature is 0.0°C. (a) What is the length of the pipe? (b) What is the fundamental frequency at a temperature of 30.0°C? (0.552m, 316Hz) 15. A stretched string is 160 cm long and has a linear density of 0.015 kg/m. What tension in the string will results in a second harmonic of 460 Hz? (810N) 16. A piano tuner strikes a 440. Hz tuning fork at the instant she strikes a piano key that should emit a tone of 440. Hz and hears a beat frequency of 2 Hz. What are the possible frequencies the piano key could be emitting? (438 Hz & 442 Hz) fb = |f2-‐f1| 440. Hz + 2 = 442 Hz 440. Hz – 2 = 438 Hz