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CU-Boulder PHYS 2020 - Relativity

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CTRelativity-1A rocket is moving to the right at speed v = (3/4)c. On the front ofthe rocket is a headlight which emits a flash of light.In the reference frame of a person at rest relative to the rocket, the speed of the light flash isA) c B) 7/4 c C) 1/4 c D) None of theseCTRelativity-2A light bulb flashes in the center of a train car that is moving at speed v with respect to the ground. In the frame of reference of thetrain car, light wave from the flash strikes the front and back of the train simultaneously. In the frame of reference of the ground, the light strikes the back ofthe train ___________ (fill in the blank) the light strikes the front of the train.A) before B) after C) at the same time asv Ground frame v = (3/4) clight beamv = 0CTRelativity-3A long row of clocks (all at rest in Frame S) are synchronized. The same row of clocks is observed from Frame S' which is moving to the left relative to S with speed v. So in S', the clocks are moving to the right at speed v. Are the clocks synchronized in S' ?A) Yes B) NoFrame SxyFrame S'x'y'vvFrame S'x'y'(B) No(A) YesCTRelativity-4Einstein showed that mass is equivalent to energy according to the formula E= mc2. The formula applies:(A) Only to nuclear reactions(B) To any kind of reaction: chemical, mechanical, gravitational, etcCTRelativity-5A light clock in its own Clock FrameEvent 1: a lightbulb flashesEvent 2: the flash hits mirror distance D away.time interval = to = D / cIn the "lab frame" the light clock is moving to the rightat speed v. Is the time interval t between events 1 and 2 in the Lab frame greater than, less than, or equal to the time interval in the Clock frameA) t > tot < toC) t = todetectorflashbulbmirrorDClock FramevDpath of light rayin lab frame12tCTRelativity-6Two Clocks, labeled A and B, are flying past each other with relative speed v. The speed is so large that the gamma factor  112vc is  = 2. . From the point of view of Bert, the little manwho lives in clock B, he is at rest and clock A is moving by at speed v. According to Albert, the little man who lives inside clock A, when one second passes on his clock A, the number of seconds that pass on Bert's clock, clock B, is..A) 1sec B) 1/2 sec C) 2 secBert's reference frame: B Bert says: "I'm at rest." A Albert vCTRelativity-7Two meter sticks, labeled L (for lab) and M (for moving) are flyingby each other at very high relative speed v. The gamma factor is  112vc=2. In the lab frame shown below, stick L is at rest and stick M is moving left at speed v. According Marvin, the little manwho lives on stick M, the length of stick L is A) 1m B) 1/2 m C) 2 mD) None of these.Lab frame: stick L stick M v Marvin.CTRelativity-8 Hydrogen and oxygen react explosively producingwater, according to the reaction:2H2 + O2  2H2O + energyTrue (A) or False (B): the mass of two H2O molecules (the product) is exactly equal to the mass of two hydrogen molecules plus the mass of one oxygen molecule (the reactants).CTRelativity-9 The hydrogen atom (1 proton + 1 electron), can exist only in certain discrete energy states corresponding to different discrete electron orbits. The lowest energy state (called the ground state) has an energy of Eground= –13.6eV. The next highest energy state state has an energy of –3.4eV. The rest mass energy of the H-atom is about 940 MeV. When a hydrogen atom undergoes a transition from the 1st excited state to the ground state, the mass of the hydrogen atom...A) remains unchanged.B) decreases by about 1 part in 107 = 1E7C) decreases by about 1 part in 108 = 1E8D) decreases by about 1 part in 109 = 1E9el ect r on gr oun d st at e 1st exci t ed st at e photonCTRelativity-10Below is a table of values of gamma. At what speed (approximately) is the kinetic energy of a particle equal to its rest mass energy?A) v = 0.75c B) v = 0.87cC) v = 0.94c D) None of these (to two place precision).v/c gamma0.1 1.00500.5 1.1550.6 1.2500.7 1.4000.8 1.6670.9 2.290.95 3.200.99 7.090.999 22.40.9999 70.70.99999 223CTRelativity-11 The following process cannot occur: An electron and a positron (an anti-electron) are placed near each other, initially at rest, and are then released. There are no other particles nearby. The electron and positron fly toward each other and annihilate, resulting in the production of a single gamma-ray with energyE m co  22 where mo is the rest mass of an electron. The process just described cannot occur because it would violate conservation of....A) energy B) momentum C) charge D) treesCTRelativity-12 In the Lab frame (in which lab equipment is at rest), a source emits a light flash, and a detector a distance D away,detects the light after a light-travel time of D/c.In the frame moving to theright at speed v, the lighttravel time is _______ ,compared to the time in theLab frame.A) greater B) lessC) the same detectorsourceDLab Frame:Moving


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CU-Boulder PHYS 2020 - Relativity

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