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Unformatted text preview: lenoir (wml297) homework 12 Turner (58220) 1 This printout should have 12 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 3) 10.0 points A student builds and calibrates an accelerom eter, which she uses to determine the speed of her car around a certain highway curve. The accelerometer is a plumb bob with a protrac tor that she attaches to the roof of her car. A friend riding in the car with her observes that the plumb bob hangs at an angle of 19 from the vertical when the car has a speed of 30 . 4 m / s. The acceleration of gravity is 9 . 8 m / s 2 . At this instant, what is the centripetal ac celeration of the car rounding the curve? Correct answer: 3 . 37441 m / s 2 . Explanation: For the plumb bob, along the vertical direc tion we have T cos = mg along the horizontal direction we have T sin = mv 2 r From these two equations the centripetal ac celeration is a c = v 2 r = g tan , the radius of the curve is r = v 2 g tan , and the speed of the car v = radicalbig g r tan . The deflection angle is 1 = 19 , so a c = g tan 1 = ( 9 . 8 m / s 2 ) tan19 = 3 . 37441 m / s 2 . 002 (part 2 of 3) 10.0 points What is the radius of the curve? Correct answer: 273 . 873 m. Explanation: The radius is r = v 2 g tan 1 = (30 . 4 m / s) 2 (9 . 8 m / s 2 ) tan19 = 273 . 873 m . 003 (part 3 of 3) 10.0 points What is the speed of the car if the plumb bobs deflection is 6 . 7 while rounding the same curve? Correct answer: 17 . 7565 m / s. Explanation: Here the deflection angle is 2 = 6 . 7 so v 2 = radicalbig g r tan 2 = radicalBig (9 . 8 m / s 2 ) (273 . 873 m) tan6 . 7 = 17 . 7565 m / s . 004 10.0 points A highway curves to the left with radius of curvature R = 48 m. The highways surface is banked at = 25 so that the cars can take this curve at higher speeds. Consider a car of mass 971 kg whose tires have static friction coefficient = 0 . 83 against the pavement. The acceleration of gravity is 9 . 8 m / s 2 . lenoir (wml297) homework 12 Turner (58220) 2 top view R = 48 m 2 5 rear view = . 8 3 How fast can the car take this curve without skidding to the outside of the curve? Correct answer: 31 . 5406 m / s. Explanation: Let : R = 48 m , = 25 , m = 971 kg , and = 0 . 83 . Basic Concepts: (1) To keep an object moving in a circle requires a net force of magnitude F c = ma c = m v 2 r directed toward the center of the circle. (2) Static friction law: F N ....
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This note was uploaded on 04/15/2010 for the course PHY 12343 taught by Professor Turner during the Spring '10 term at University of Texas at Austin.
 Spring '10
 Turner

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