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Unformatted text preview: oldhomewk 25 YOO, HEE Due: Apr 1 2008, 4:00 am 1 Question 1, chap 13, sect 1. part 1 of 1 10 points A rod can pivot at one end and is free to rotate without friction about a vertical axis, as shown. A force vector F is applied at the other end, at an angle to the rod. L m F If vector F were to be applied perpendicular to the rod, at what distance d from the axis of rotation should it be applied in order to produce the same torque vector ? 1. d = L sin correct 2. d = L 3. d = 2 L 4. d = L tan 5. d = L cos Explanation: The torque the force generates is = F L sin . Thus the distance in question should be L sin . Question 2, chap 13, sect 1. part 1 of 4 10 points A 2700 kg block is lifted at a constant speed of 5 cm / s by a steel cable that passes over a massless pulley to a motor-driven winch. The radius of the winch drum is 20 cm and the winch drum has a mass of 1500 kg. The acceleration due to gravity is 9 . 81 m / s 2 . 2700 kg 2 c m 5cm / s 1500 kg What force must be exerted by the cable? Correct answer: 26 . 487 kN (tolerance 1 %). Explanation: Let : m = 2700 kg , v = 5 cm / s = 0 . 05 m / s , and r = 20 cm = 0 . 2 m . Note: There is no acceleration in this sys- tem and the mass of the winch drum is not required. Because the block is lifted at constant speed, the cable tension is T = mg = (2700 kg) (9 . 81 m / s 2 ) kN N = 26 . 487 kN . Question 3, chap 13, sect 1. part 2 of 4 10 points What torque does the cable exert on the winch drum? Correct answer: 5 . 2974 kN m (tolerance 1 %). Explanation: The torque is = T r = (26 . 487 kN) (0 . 2 m) = 5 . 2974 kN m ....
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