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Unformatted text preview: pampalone (tp8327) HW#8 swaminathan (10500) 1 This printout should have 19 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points A penny of mass 3 . 1 g rests on a small 8 . 36 g block supported by a spinning disk of radius 23 cm. The coefficients of friction between block and disk are 0 . 795 (static) and 0 . 64 (kinetic) while those for the penny and block are 0 . 58 (static) and 0 . 45 (kinetic). Disk Penny Block r What is the maximum speed of the disk without the block and penny sliding? The acceleration of gravity is 9 . 8 m / s 2 . Correct answer: 47 . 4717 rpm. Explanation: Let : = 0 . 58 , r = 23 cm , and g = 9 . 8 m / s 2 . The frictional force provides the centripetal force mg = mv 2 r , so the speed is v = g r = r = radicalbigg g r = radicalbigg . 58 (9 . 8 m / s 2 ) . 23 m 60 s 1 min 1 rev 2 = 47 . 4717 rpm . 002 10.0 points A coin is placed 25 cm from the center of a horizontal turntable, initially at rest. The turntable then begins to rotate. When the speed of the coin is 120 cm / s (rotating at a constant rate), the coin just begins to slip. The acceleration of gravity is 980 cm / s 2 . What is the coefficient of static friction be tween the coin and the turntable? Correct answer: 0 . 587755. Explanation: The normal force on the coin is N = mg . The force provided by friction immediately before slippage is given by f = N = mg . At this moment the centripetal acceleration (there is no tangential acceleration) provides a force F = mv 2 r . Summation of forces yields F = f = mg = mv 2 r , therefore = v 2 r g = (120 cm / s) 2 (25 cm) (980 cm / s 2 ) = 0 . 587755 . 003 10.0 points A man is riding a looptheloop type roller coaster, and he is sitting on a spring scale as the car moves. The car rolls freely on the circular track, and thus moves fastest at the bottom of the loop and slowest at the top. A B C D pampalone (tp8327) HW#8 swaminathan (10500) 2 In which position of the car and man is the reading of the spring scale largest ? 1. D 2. B 3. C 4. A correct Explanation: At the lowest position, the only force avail able pointing upward on the man is the sup porting force exerted by the scale. Since grav ity acts downward, that force must be much greater than it is in any other of the positions, where gravity is either perpendicular to vectorn (at positions B and D) or in the same direction as vectorn (at position C). 004 10.0 points Given: g = 9 . 8 m / s 2 . A small glass of water is placed on carousel inside a microwave oven, at a radius 10 cm from the center. The carousel rotates steadily, tuning through one revolution every 3 . 5 s....
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 Spring '08
 KEN
 Mass

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