This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: billing (cab4763) HW 7 opyrchal (11109) 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) 716 kg elevator starts from rest. It moves upward for 4 . 36 s with a constant acceleration until it reaches its cruising speed of 2 . 14 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: 7 . 88401 kW. Explanation: Let : m = 716 kg , t = 4 . 36 s , and v = 2 . 14 m / s , The height y is y = v ave t = 1 2 v t = 1 2 (2 . 14 m / s) (4 . 36 s) = 4 . 6652 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 ) = (716 kg) (2 . 14 m / s) 2 (4 . 36 s) [(9 . 8 m / s 2 ) (4 . 36 s) + 2 . 14 m / s] = 7884 . 01 W = 7 . 88401 kW . 002 10.0 points A block of mass 0 . 19 kg is placed on a verti cal spring of constant 6399 N / m and pushed downward, compressing the spring 0 . 15 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: 38 . 6621 m. Explanation: Given : m = 0 . 19 kg , x = 0 . 15 m , k = 6399 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 = (6399 N / m)(0 . 15 m) 2 2(0 . 19 kg)(9 . 8 m / s 2 ) = 38 . 6621 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 . 9 m as in the figure below. The sphere is released from rest when the cord makes a 51 angle with the vertical, and the pivot at...
View
Full
Document
This note was uploaded on 11/23/2010 for the course PHYS PHYS111 taught by Professor Gayen during the Spring '10 term at NJIT.
 Spring '10
 gayen
 Physics

Click to edit the document details