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Unformatted text preview: pampalone (tp8327) HW#8 swaminathan (10500) 1 This print-out should have 19 questions. Multiple-choice 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 loop-the-loop 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