However, although Sonic now has movie star credentials, he’s entered a whole new world of superpowers, meta-humans and professional speedsters, who operate on another level entirely. The spiny blue speedster, formerly confined to video games, animated shows and pencil cases, is now standing toe-to-toe with the bona fide blockbuster big league. (credit: U.S.Although his movie dragged its feet somewhat getting into cinemas following a three-month delay, Sonic the Hedgehog is one of the fastest characters in all of pop culture. Such a bow wake is called Cerenkov radiation and is commonly observed in particle physics.įigure 17.19 The blue glow in this research reactor pool is Cerenkov radiation caused by subatomic particles traveling faster than the speed of light in water. If the particle creates light in its passage, that light spreads on a cone with an angle indicative of the speed of the particle, as illustrated in Figure 17.19. We know that wavelength and frequency are related by v w = fλ v w = fλ size 12 in the medium of water, the speed of light is closer to 0.75 c 0.75 c. Motion away from the source decreases frequency as the observer on the left passes through fewer wave crests than he would if stationary. Motion toward the source increases frequency as the observer on the right passes through more wave crests than she would if stationary. The observer moving toward the source receives them at a higher frequency, and the person moving away from the source receives them at a lower frequency.įigure 17.15 The same effect is produced when the observers move relative to the source. Finally, if the observers move, as in Figure 17.15, the frequency at which they receive the compressions changes. Thus, the wavelength is shorter in the direction the source is moving (on the right in Figure 17.14), and longer in the opposite direction (on the left in Figure 17.14). This moving emission point causes the air compressions to be closer together on one side and farther apart on the other. Each compression of the air moves out in a sphere from the point where it was emitted, but the point of emission moves. If the source is moving, as in Figure 17.14, then the situation is different. If the source is stationary, then all of the spheres representing the air compressions in the sound wave centered on the same point, and the stationary observers on either side see the same wavelength and frequency as emitted by the source, as in Figure 17.13. Each disturbance spreads out spherically from the point where the sound was emitted. What causes the Doppler shift? Figure 17.13, Figure 17.14, and Figure 17.15 compare sound waves emitted by stationary and moving sources in a stationary air mass. Their music was observed both on and off the train, and changes in frequency were measured. Doppler, for example, had musicians play on a moving open train car and also play standing next to the train tracks as a train passed by. The Doppler effect and Doppler shift are named for the Austrian physicist and mathematician Christian Johann Doppler (1803–1853), who did experiments with both moving sources and moving observers. The actual change in frequency due to relative motion of source and observer is called a Doppler shift.
For example, if you ride a train past a stationary warning bell, you will hear the bell’s frequency shift from high to low as you pass by. Although less familiar, this effect is easily noticed for a stationary source and moving observer. The Doppler effect is an alteration in the observed frequency of a sound due to motion of either the source or the observer. It is so familiar that it is used to imply motion and children often mimic it in play. We also hear this characteristic shift in frequency for passing race cars, airplanes, and trains. The faster the motorcycle moves, the greater the shift. The closer the motorcycle brushes by, the more abrupt the shift. The high-pitch scream shifts dramatically to a lower-pitch roar as the motorcycle passes by a stationary observer. The characteristic sound of a motorcycle buzzing by is an example of the Doppler effect.
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