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Police radar determines the speed of motor vehicles using the same Doppler-shift technique employed for ultrasound in medical diagnostics. Beats are produced by mixing the double Doppler-shifted echo with the original frequency. If
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Physics: Principles with Applications
- Radio station WWVB, operated by the National Institute of Standards and Technology (NIST) from Fort Collins, Colorado, at a low frequency of 60 kHz, broadcasts a time synchronization signal whose range covers the entire continental US. The timing of the synchronization signal is controlled by a set of atomic clocks to an accuracy of 101012 s, and repeats every 1 minute. The signal is used for devices, such as radio-controlled watches, that automatically synchronize with it at preset local times. WWVB's long wavelength signal tends to propagate close to the ground. (a) Calculate the wavelength of the radio waves from WWVB. (b) Estimate the error that the travel time of the signal causes in synchronizing a radio controlled watch in Norfolk, Virginia, which is 1570 mi (2527 km) from Fort Collins, Colorado.arrow_forwardAn astronomer measures the speed of recession of a remote galaxy to be 365 km/s using the Doppler principle According to the Hubble relation, about how far away is the galaxy?arrow_forward(a) Seismographs measure the arrival times of earthquakes with a precision of 0.100 s. To get the distance to the epicenter of the quake, geologists compare the arrival times of S- and P-waves, which travel at different speeds. If S- and P-waves travel at 4.00 and 7.20 km/s, respectively, in the region considered, how precisely can the distance to the source of the earthquake be determined? (b) Seismic waves from underground detonations of nuclear bombs can be used to locate the test site and detect violations of test bans. Discuss whether your answer to (a) implies a serious limit to such detection. (Note also that the uncertainty is greater if there is an uncertainty in the propagation speeds of the S- and P-waves.)arrow_forward
- A device called an insolation meter is used to measure the intensity of sunlight. It has an area of 100 cm2 and registers 6.50 W. What is the intensity in W/m2?arrow_forwardRadio waves transmitted through space at 3.00108m/s by the Voyager spacecraft have a wavelength of 0.120 m. What is their frequency?arrow_forwardRadio waves transmitted through empty space at the speed of light (v=c=3.00108m/s) by the Voyager spacecraft have a wavelength of 0.120 m. What is their frequency?arrow_forward
- When poked by a spear, an operatic soprano lets out a 1200—Hz shriek. What is its wavelength if the speed of sound is 345 m/s?arrow_forwardPolice radar detects the speed of a car (Fig. P38.15) as follows. Microwaves of a precisely known frequency are broadcast toward the car. The moving car reflects the microwaves with a Doppler shift. The reflected waves are received and combined with an attenuated version of the transmitted wave. Beats occur between the two microwave signals. The beat frequency is measured. (a) For an electromagnetic wave reflected back to its source from a mirror approaching at speed v, show that the reflected wave has frequency f=c+vcvf where f is the source frequency. (b) Noting that v is much less than c, show that the beat frequency can be written as fbeat = 2v/. (c) What beat frequency is measured for a car speed of 30.0 m/s if the microwaves have frequency 10.0 GHz? (d) If the beat frequency measurement in part (c) is accurate to 5.0 Hz, how accurate is the speed measurement?arrow_forwardAn entrepreneur decides to invent and market a device that will fool the Doppler radar units used to detect cars that are speeding. The device would be placed at the very front of the car and would detect the radar signal, determine its frequency, and transmit back its own radar signal that would make the radar unit register a legal speed. What would the frequency of the "fake'' signal have 10 be in comparison to the original? If a second unit were designed to be placed at the back end of the car, what would be different about the frequency it would have to use compared to that used by the unit at the from?arrow_forward
- A sound wave propagates in air at 27C with frequency 4.00 kHz. It passes through a region where the temperature gradually changes and then moves through air at 0C. Give numerical answers to the following questions to the extent possible and state your reasoning about what happens to the wave physically. (a) What happens to the speed of the wave? (b) What happens to its frequency? (c) What happens to its wavelength?arrow_forwardSubmarine A travels horizontally at 11.0 m/s through ocean water. It emits a sonar signal of frequency f = 5.27 103 Hz in the forward direction. Submarine B is in front of submarine A and traveling at 3.00 m/s relative to the water in the same direction as submarine A. A crewman in submarine B uses his equipment to detect the sound waves (pings) from submarine A. We wish to determine what is heard by the crewman in submarine B. (a) An observer on which submarine detects a frequency f as described by Equation 16.46? (b) In Equation 16.46, should the sign of vs be positive or negative? (c) In Equation 16.46, should the sign of vo be positive or negative? (d) In Equation 16.46, what speed of sound should be used? (e) Find the frequency of the sound detected by the crewman on submarine B.arrow_forwardMedical Application (a) What is the intensity in W/m2 of a laser beam used to burn away cancerous tissue that, when 90.0% absorbed, puts 500 J of energy into a circular spot 2.00 mm in diameter in 4.00 s? (b) Discuss how this intensity compares to the average intensity of sunlight (about 700W/m2 ) and the implications that would have if the laser beam entered your eye. Note how your answer depends on me time duration of the exposure.arrow_forward
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