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Unformatted text preview: FORM 0 UM Physics Competition 13 Apr 2002 When you need a numerical value for g , use 10 m/s 2 . [1] Which of the following electromagnetic waves is not emitted as a result of changes in the energy level of an electron in an atom? [A] gamma radiation. [B] ultraviolet radiation. [C] infrared radiation. [D] visible light. [E] sound. Solution: Sound isn’t even an electromagnetic wave. The others are emitted when the electrons make transitions. [A] [2] The two most abundant elements in the sun are [A] hydrogen and helium. [B] oxygen and carbon. [C] carbon and hydrogen. [D] nitrogen and oxygen. [E] hydrogen and oxygen. Solution: The sun is about 75% hydrogen and about 25% helium, with a little bit of some heavier elements. [A] [3] A helicopter is rising straight up at a constant speed of 5 m/s. When it is 30 meters above the ground a package is let go from a window. How long does it take for the package to reach the ground? [A] 3 sec [B] 0 . 5 sec [C] 2 sec [D] 2 . 45 sec [E] 6 sec Solution: y = v t gt 2 / 2, so 30 = 5 t 5 t 2 (in meters and seconds). This is t 2 t 6 = ( t 3)( t + 2) = 0, so t = 3 s [A] [4] At night approximately 500 photons per second must enter an unaided human eye for an object to be seen. A light bulb emits about 5 . 00 × 10 18 photons per second uniformly in all directions. The radius of the pupil of the eye is about 4 × 10 − 3 meters. What is the maximum distance from which the bulb could be seen? [A] 2 . × 10 4 m [B] 5 . × 10 7 m [C] 2 . × 10 5 m [D] 20 m [E] 5 . × 10 3 m Solution: Photons per area per second at a distance r are 5 . 00 × 10 18 / 4 πr 2 . Photons per second entering the eye, radius R is then this times πR 2 . Set this product equal to 500 per second and solve for r . The result is [A] [5] A small engine, during each cycle, takes in 16,000 joules of heat energy, and rejects 12,000 joules in the exhaust. What is the maximum possible efficiency of the engine? [A] 25% [B] 75% [C] 33% [D] 67% [E] 100% Solution: 16 , 000 12 , 000 = 4 , 000 Joules is available for doing work. The best you can get for the efficiency is then 4000 / 16000 = 0 . 25 for the efficiency. [A] [6] What is the direction of the magnetic field if an electron initially moving to the East is deflected horizontally toward the South? [A] down [B] up [C] East [D] West [E] North [F] South Solution: vector F = qvectorv × vector B . The electron has negative charge. East × Down is North: [A] FORM 0 [7] An automobile of mass m is going around a curve in an arc of a circle of radius R at a speed v . The curve is banked at an angle θ to the horizontal and the coefficient of static friction between the tires and the road is μ s . If θ is not very big, the maximum speed the car can be moving without skidding is? Hint: You don’t have to solve it, you can analyze the answers and eliminate four of them....
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This note was uploaded on 01/10/2011 for the course PHYS 100 taught by Professor Razasuleman during the Spring '10 term at University of Engineering & Technology.
 Spring '10
 RazaSuleman
 Physics, Energy, Radiation

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