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Unformatted text preview: gutierrez (ig3472) HW5 ditmire (58216) 1 This printout should have 26 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 (part 1 of 2) 10.0 points A spring has a force constant of 311 N / m and an unstretched length of 7 cm. One end is attached to a post that is free to rotate in the center of a smooth table, as shown in the top view below. The other end is attached to a 9 kg disk moving in uniform circular motion on the table, which stretches the spring by 1 cm. Note: Friction is negligible. 311 N / m 9 kg 8 cm What is the centripetal force F c on the disk? Correct answer: 3 . 11 N. Explanation: Let : r = 7 cm = 0 . 07 m , r = 1 cm = 0 . 01 m , m = 9 kg , and k = 311 N / m . The centripetal force is supplied only by the spring. Given the force constant and the extension of the spring, we can calculate the force as F c = k r = (311 N / m) (0 . 01 m) = 3 . 11 N . 002 (part 2 of 2) 10.0 points What is the work done on the disk by the spring during one full circle? 1. W = 0 . 00106673 J 2. W = 0 J correct 3. W = 1 . 36785 J 4. W = 0 . 2177 J 5. W = 0 . 683925 J Explanation: Since the force is always perpendicular to the movement of the disk, the work done by the spring is zero . 003 (part 1 of 3) 10.0 points A 1270 kg car accelerates uniformly from rest to 14 . 2 m / s in 4 . 02 s. Find the work done on the car in this time interval. Correct answer: 128 . 041 kJ. Explanation: Given : m = 1270 kg , v f = 14 . 2 m / s , and t = 4 . 02 s . W = K = 1 2 mv 2 f = 1 2 (1270 kg) (14 . 2 m / s) 2 1 kJ 1000 J = 128 . 041 kJ . 004 (part 2 of 3) 10.0 points Find the average power delivered by the en gine in this time interval. Correct answer: 42 . 6958 hp. Explanation: P = W t gutierrez (ig3472) HW5 ditmire (58216) 2 = parenleftbigg 128 . 041 kJ 4 . 02 s parenrightbiggparenleftbigg 1000 J kJ parenrightbiggparenleftbigg 1 hp 746 W parenrightbigg = 42 . 6958 hp . 005 (part 3 of 3) 10.0 points Find the instantaneous power delivered by the engine at t = 1 . 84 s. Correct answer: 39 . 0847 hp. Explanation: Given : t 1 = 1 . 84 s In the first 4 . 02 s a = v t = v f v i t = v f t since v i = 0, so the net force is F = ma = mv f t . At t = 1 . 84 s , the instantaneous velocity is v = v i + a t 1 = a t 1 = v f t t 1 since v i = 0 , and the power output is P = F v = mv f t 1 parenleftBig v f t t 1 parenrightBig = m parenleftBig v f t parenrightBig 2 t 1 = (1270 kg) parenleftbigg 14 . 2 m / s 4 . 02 s parenrightbigg 2 (1 . 84 s) parenleftbigg 1 hp 746 W parenrightbigg = 39 . 0847 hp . 006 (part 1 of 5) 10.0 points A crate is pulled up a rough incline. The pulling force is parallel to the incline. The crate is pulled a distance of 5 . 04 m....
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 Spring '08
 Turner
 Physics, Force

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