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Unformatted text preview: sindt (as53996) HW 7 opyrchal (11101) 1 This printout should have 8 questions. Multiplechoice questions may continue on the next column or page find all choices before answering. 001 10.0 points A(n) 708 kg elevator starts from rest. It moves upward for 3 . 07 s with a constant acceleration until it reaches its cruising speed of 1 . 81 m / s. The acceleration of gravity is 9 . 8 m / s 2 . Find the average power delivered by the elevator motor during the period of this accel eration. Correct answer: 6 . 65702 kW. Explanation: Let : m = 708 kg , t = 3 . 07 s , and v = 1 . 81 m / s , The height y is y = v ave t = 1 2 v t = 1 2 (1 . 81 m / s) (3 . 07 s) = 2 . 77835 m . Since the elevator starts from rest, the power P supplied by the motor, totally transferred into Kinetic Energy K and Potential Energy U , where E = K + U , is P = E t = U + K t = mg y + 1 2 mv 2 t = mg 1 2 v t + 1 2 mv 2 t = mv 2 t ( g t + v ) = (708 kg) (1 . 81 m / s) 2 (3 . 07 s) [(9 . 8 m / s 2 ) (3 . 07 s) + 1 . 81 m / s] = 6657 . 02 W = 6 . 65702 kW . 002 10.0 points A block of mass 0 . 19 kg is placed on a verti cal spring of constant 2370 N / m and pushed downward, compressing the spring 0 . 14 m. After the block is released it leaves the spring and continues to travel upward. The acceleration of gravity is 9 . 8 m / s 2 . What height above the point of release will the block reach if air resistance is negligible? Correct answer: 12 . 4737 m. Explanation: Given : m = 0 . 19 kg , x = 0 . 14 m , k = 2370 N / m , and g = 9 . 8 m / s 2 . Choose U g = 0 at the level of the release point. K i = K f = 0 , so from conservation of en ergy, ( U g + U s ) i = ( U g + U s ) f 0 + U s,i = U g,f + 0 1 2 k x 2 = mg h h = k x 2 2 mg = (2370 N / m)(0 . 14 m) 2 2(0 . 19 kg)(9 . 8 m / s 2 ) = 12 . 4737 m . 003 (part 1 of 2) 10.0 points A pendulum consists of a sphere of mass 1 . 9 kg attached to a light cord of length 10 . 8 m as in the figure below. The sphere is released from rest when the cord makes a 41 . 8 angle with the vertical, and the pivot at...
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 Spring '09
 WANG
 Physics

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