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Unformatted text preview: sanchez (ds28677) – homework 23 – Turner – (58220) 1 This printout should have 11 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Two pulley wheels, of respective radii R 1 = . 15 m and R 2 = 1 . 2 m are mounted rigidly on a common axle and clamped together. The combined moment of inertia of the two wheels is I + 1 . 6 kg m 2 . Mass m 1 = 32 kg is attached to a cord wrapped around the first wheel, and another mass m 2 = 8 . 7 kg is attached to another cord wrapped around the second wheel: 1 2 m R 1 m R 2 Find the angular acceleration of the system. Take clockwise direction as positive. The acceleration of gravity is 9 . 8 m / s 2 . Correct answer: 3 . 72252 rad / s 2 . Explanation: 1 2 m g R 1 m g R 2 T 1 2 T 2 T T 1 Let’s assume m 2 moves downward and take motion downward as positive for m 2 and mo tion upward as positive for m 1 , take clockwise as positive for the pulley wheels. Since the two masses are connected to the pulleys, their accelerations are a 1 = αR 1 and a 2 = αR 2 . Applying Newton’s second law to m 1 , m 2 , and the pulleys separately, we obtain T 1 m 1 g = m 1 a 1 = m 1 αR 1 (1) m 2 g T 2 = m 2 a 2 = m 2 αR 2 (2) τ net = T 2 R 2 T 1 R 1 = I α. (3) Solving these equations, we find α = m 2 g R 2 m 1 g R 1 I + m 1 R 2 1 + m 2 R 2 2 = 3 . 72252 rad / s 2 . The fact that α > 0 indicates that our pre vious assumption that m 2 moves downward is right. If α < 0, our assumption was not correct and m 2 would move upward, but the equations obtained to calculate T 1 and T 2 would be still correct and we need not reas sume the direction of the motion and recalcu late. 002 10.0 points A bicycle wheel has a diameter of 68 . 1 cm and a mass of 1 . 44 kg. The bicycle is placed on a stationary stand and a resistive force of 156 N is applied tangent to the rim of the tire. Assume that the wheel is a hoop with all of the mass concentrated on the outside radius. In order to give the wheel an acceleration of 4 . 01 rad / s 2 , what force must be applied by a chain passing over a 1 . 7 cm diameter sprocket? Correct answer: 6327 . 94 N. Explanation: Let : r w = 34 . 05 cm = 0 . 3405 m , M = 1 . 44 kg , F f = 156 N , r s = 0 . 85 m = 0 . 0085 m , and α = 4 . 01 rad / s 2 . For a wheel with mass concentrated on the rim, I = M R 2 . Applying rotational equilib rium summationdisplay τ = I α F r F f R = I α sanchez (ds28677) – homework 23 – Turner – (58220) 2 F = F f R + I α r = F f R + M R 2 α r = (156 N)(0 . 3405 m) . 0085 m + (1 . 44 kg)(0 . 3405 m) 2 (4 . 01 rad / s 2 ) . 0085 m = 6327 . 94 N ....
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This note was uploaded on 04/07/2011 for the course PHY 303K taught by Professor Turner during the Spring '08 term at University of Texas.
 Spring '08
 Turner
 Physics, Work

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