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Unformatted text preview: kuruvila (lk5992) HW 7 opyrchal (11113) 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) 769 kg elevator starts from rest. It moves upward for 2 . 93 s with a constant acceleration until it reaches its cruising speed of 2 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: 8 . 06112 kW. Explanation: Let : m = 769 kg , t = 2 . 93 s , and v = 2 m / s , The height y is y = v ave t = 1 2 v t = 1 2 (2 m / s) (2 . 93 s) = 2 . 93 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 ) = (769 kg) (2 m / s) 2 (2 . 93 s) [(9 . 8 m / s 2 ) (2 . 93 s) + 2 m / s] = 8061 . 11 W = 8 . 06112 kW . 002 10.0 points A block of mass 0 . 25 kg is placed on a verti cal spring of constant 2188 N / m and pushed downward, compressing the spring 0 . 07 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: 2 . 188 m. Explanation: Given : m = 0 . 25 kg , x = 0 . 07 m , k = 2188 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 = (2188 N / m)(0 . 07 m) 2 2(0 . 25 kg)(9 . 8 m / s 2 ) = 2 . 188 m . 003 (part 1 of 2) 10.0 points A pendulum consists of a sphere of mass 1 . 3 kg attached to a light cord of length 11 . 1 m as in the figure below. The sphere is released from rest when the cord makes a 48 . 7 angle with the vertical, and the pivot at P is frictionless.is frictionless....
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 Fall '08
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