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UW-Madison PHYSICS 107 - Examination

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Physics 107: Ideas of Modern Physics Exam 2 March 8, 2006 Name______________________________________________________ ID #_________________________ Section #______________ On the Scantron sheet, 1) Fill in your name 2) Fill in your student ID # (not your social security #) 3) Fill in your section # (under ABC of special codes) Useful constants: c= speed of light = 3x108 m/s sound speed in air = 340 m/s g= accel. of gravity on Earth = 10 m/s2 G= gravitational constant = 6.7x10-11 N-m2/kg22 2 1. B 2. A 1360 Hz tone is generated by a speaker. What is the wavelength of the sound wave? a. 2.0 m b. 0.50 m c. 1.0 m d. 0.25 m e. 3.0 m 3. A sound wave is: a. an electromagnetic wave. b. a transverse wave. c. a radio wave. d. a longitudinal wave. e. a frequency wave. 4. In your room you have two speakers in different corners. You learned about interference in Physics 107, and positioned the speakers so that at your desk you are exactly 2 meters from each. Your roommate moved one of your speakers: you now hear a cancellation of sound at 680 Hz. How far did your roommate move your speaker toward your desk? a. 0.5 m b. 0.25 m c. 1.0 m d. 0.12 m e. 0.1 m3 3 5. Your Badger radio network broadcasts on AM 1310. This frequency is 1,310 kHz (1 kHz = 1000 Hz). How many football fields long is one wavelength (1 football field ~ 91 meters)? a. 0.25 b. 0.1 c. 1.1 d. 0.4 e. 2.5 6. An observed source of magnetic fields is: a. magnetic currents b. magnetic monopoles c. voltage currents d. electric charges e. electric currents 7. The force between two charged particles 0.4 m apart is found to be repulsive, with magnitude 2x10-10 N. What is the force after the charge on each particle is doubled, with the particles still 0.4 m apart? a. 4.0x10-10 N, repulsive b. 4.0x10-10 N, attractive. c. 0.5x10-10 N, attractive. d. 8.0x10-10 N, repulsive e. 1.0x10-10 N, repulsive 8. Suppose an eye has only two cones with spectral sensitivities shown here. Which combinations of lights will give the same color perception? a. B&D same as C b. A&E same as C c. B&E same as D d. A&C same as D e. B&C same as D 300 nm 400 nm 500 nm 600 nm A B C D E4 4 9. Your microwave beams 2 Giga-Hertz (2x109 Hz) microwave radiation at the food inside of it. It does this because water preferentially absorbs radiation at that frequency. The oven can enhance this effect by heating the food in a resonant cavity, designed so that an integer number of wavelengths fit inside its length. What should the length of the oven be if two wavelengths fit inside? A. 6.7 m B. 0.5 m C. 0.67 m D. 1.3 m E. 0.30 m 10. An electron moves continuously up and down in one corner of room. An experimenter in the opposite corner of the room measures at his location a. an unchanging electric field.. b. an electromagnetic wave c. a charge. d. an unchanging magnetic field. e. a current. 11. Apollo 15 astronauts placed a laser reflector on the moon. A laser beam from Earth reflects for this. The round trip time (to and from the moon) for the laser beam is found to be 2.5 seconds. How far away was the moon at this time? a. 7.5 x 108 m b. 850 m c. 3.75 x 106 m d. 3.75 x 108 m e. 425 m 12. Einstein’s special theory of relativity is based on which one of the following postulates? a. Space and time are absolutes. b. The speed of light is constant. c. Energy is always conserved. d. Photons have zero mass. e. Space-time is curved.5 5 13.An Earth observer sees two spaceships approaching each other. One moves at 0.2c, the other at 0.3c relative to the Earth observer. The 0.2c one flashes it’s headlights. The other spaceship measures the light pulse from those headlights passing at A. 1.2c B. 1.5c C. 0.6c D. 1.0c E. 0c 14. A car is 6 meters long when it is at rest. It drives by at 0.6c relative to an observer on the street corner. The street corner observer measures the car to have a length of: a. 7.5 m b. 4.8 m c. 5.7 m d. 0.6 m e. 2.1 m 15. An astronaut travels by the Earth at 0.6c. Someone stationary on Earth holds a meter stick. The astronaut measures the lengths of this meter stick to be. A. More than one meter. B. Less than one meter. C. Need to know Earth velocity. D. Exactly one meter. E. Depends on Earth reference frame.6 6 16.A 25 year-old astronaut travels to a nearby star and back at a speed of 0.8c, leaving behind her twin on Earth. The Earth twin measures the star to be 8 Light-Years away. When the astronaut returns, how does her biological age compare to the Earth twin? A. 6.67 years younger B. 10 years older C. 4 years younger D. 5 years younger E. 4 years older 17. An astronaut in a spaceship moves relative to an Earth observer. The Earth observer measures the time interval between heart beats of the astronaut. As the speed of the astronaut gets closer and closer to the speed of light (assume the astronaut does not get excited), the earth observer sees that the time between the astronaut’s heart beats A. decreases, getting arbitrarily close to zero. B. increases without bound C. decreases, approaching c/10. D. increases, but never exceeds c, the speed of light. E. increases for positive speeds, decreases for negative speeds. 18. The equivalence principle of general relativity says that A. energy and mass are equivalent, and can be transformed to each other. B. space and time are equivalent, and should be plotted together. C. gravity is equivalent to an accelerating reference frame without gravity. D. time dilation is equivalent to length contraction. E. all space-time metrics are equivalent.7 7 19. As the speed of a massive particle approaches the speed of light, the relativistic mass of the particle a. does not change. b. approaches the rest mass. c. increases slightly. d. increases without bound. e. decreases to zero. 20. The first atomic bomb exploded with an energy of 15 kilotons (15,000 tons of TNT). (1 kiloton = 4.184 x 1012 Joules). If this were accomplished by converting mass directly into energy, how much mass would have been required (1000 gram = 1 kilo-gram)? a. 310,000 kg b. 1400 kg c. 0.7 gm d. 700 gm e. 0.005 gm 21. General relativity would say a dropped ball accelerates toward the Earth because A. of the gravitational force from Earth. B. the speed of light is same in all reference frames. C. the ball follows the shortest path in


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UW-Madison PHYSICS 107 - Examination

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