Physics 107: Ideas of Modern Physics Exam 2 Oct. 25, 2005 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. A 2. A 1020 Hz tone is generated by a speaker. What is the wavelength of the sound wave? A. 0.10 m B. 3.0 m C. 1.0 m D. 0.50 m E. 0.33 m 3. An example of a longitudinal wave is A. a water wave. B. a sound wave. C. an electromagnetic wave. D. a wave on a rope. E. a frequency wave. 4. In your room you have two speakers in different corners. Sitting at your desk, you are 2 meters from one of the speakers, and 2.25 meters from the other. At which frequency would the sound waves you hear tend to cancel (total sound is quieter)? A. 340 Hz B. 680 Hz C. 1360 Hz D. 170 Hz E. 250 Hz 5. Two wave pulses on a rope approach each other as shown to the right. Some time later, the rope looks like: B. C. D. E. A.3 3 6. An electric field: A. always appears in conjunction with a magnetic field. B. cannot change in time. C. cannot cause a force on a charged particle. D. can arise from electric charges. E. can arise from stationary magnets. 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 when the separation is reduced to 0.2 m? a. 4.0x10-10 N, attractive. b. 4.0x10-10 N, repulsive c. 8.0x10-10 N, repulsive d. 0.5x10-10 N, attractive. e. 1.0x10-10 N, repulsive 8. Suppose an eye has only two cones with spectral sensitivities shown here. It is stimulated by equal intensities of 300 and 700 nm pure spectral light. Which single wavelength might produce a similar color perception? a. 330 nm b. 430 nm c. 500 nm d. 530 nm e. 630 nm 9. Your microwave oven beams electromagnetic radiation at the food inside, and the food warms by absorbing the radiation. The oven is approximately a resonant cavity, with one wavelength of microwave radiation fitting inside the length. If the oven has a length of 15 cm (0.15 m), find the operating frequency. (1 Giga-Hertz=109 Hz) A. 2.0 Giga-Hertz B. 1.0 Giga-Hertz C. 0.1 Giga-Hertz D. 0.2 Giga-Hertz E. 20 Giga-Hertz4 4 10. A continuous beam of electrons moving at a constant velocity is near one corner of an otherwise empty room. At a particular location in the opposite corner of the room, an experimenter measures a. an electromagnetic wave. b. an unchanging electric field. c. an unchanging magnetic field. d. a charge. e. a current. 11. A radio transmission from an astronaut on the moon will be received how much later on the Earth? (The moon is 380 million meters from the Earth). a. 0.12 s b. 0.25 s c. 0.38 s d. 1.27 s e. 0.80 s 12. The conclusions of special relativity arise primarily from the assumption that A. nothing can exceed the speed of light. B. the speed of light is constant in all inertial reference frames. C. the ether provides a relative inertial reference frame. D. only relative velocities between inertial reference frames are important. E. space-time has a relative curvature. 13.One space ship is catching up to and passing another on the same trip between two stars. The passing ship moves at 0.9c and the other ship at 0.45c. The passing ship flashes its headlights before passing. An observer on the slower ship measures the light from the headlights to be moving at what speed relative to the slower ship? A. 1.0c B. 0.45c C. 0.0c D. 1.45c E. 1.67c5 5 14.A future personal ship is 12 meters long when it is parked at a street corner. After cruising around the block, it moves at a constant velocity of 0.8c past an observer hanging onto a light post on that same street corner. The street corner observer measures the ship to have a length of: A. 20.0 m B. 5.7 m C. 0.6 m D. 7.2 m E. 14.3 m 15. Two astronauts, Joe and Jane, are playing catch on a spaceship traveling at 0.4c relative to an Earth observer. Joe, Jane, and the Earth observer measure the time for the ball to travel from Joe to Jane and then back to Joe again. Who measures the shortest time? A. Joe. B. Jane. C. Both Joe and Jane. D. The Earth observer. E. All of them measure the same time. 16. An astronaut on a spaceship traveling from earth to mars at 0.6c has a perfect atomic clock. She uses her clock to measure the time it takes her to travel from the earth to mars and finds 10 minutes. An observer on the Earth also times the spaceship and finds a trip time of: A. 12.5 min B. 10.0 min C. 8.0 min D. 6.4 min E. 25 min6 6 17. An astronaut in a spaceship moves relative to an Earth observer. The Earth observer measures the heart rate (beats/sec) 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 astronaut’s heart rate in beats/sec A. increases without bound. B. decreases, and approaches zero. C. increases, but never gets above a maximum allowed value. D. decreases, but never gets less than a nonzero minimum value. E. depends on whether the speed is negative or positive. 18. The equivalence principle of general relativity says that A. space and time are equivalent, and should be plotted together. B. energy and mass are equivalent, and can be transformed to each other. C. time dilation is equivalent to length contraction. D. gravity is equivalent to an accelerating reference frame without gravity. E. all space-time metrics are equivalent. 19. As a particle of rest mass mo approaches the speed of light, its total energy A. approaches moc2. B. approaches zero. C. increases without bound. D. becomes entirely kinetic. E. is trapped in a black hole.7 7 20. My daughter’s blow drier uses 1000 Watts of power. If this could be powered by mass energy, how much mass would be required to run it for one year (~30 million seconds)? (Remember that 1,000,000 milligrams = 1 kg) A. 0.3 milligrams B. 0.1 milligrams C. 1 milligram D. 30 milligrams E. 3 milligrams 21. General relativity would say that two balls of different masses accelerate at the same rate toward the Earth because A.
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