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Unformatted text preview: kaplan (hmk378) – Homework 5 – Weathers – (17104) 1 This printout should have 11 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 2) 10.0 points A particle has vectorr (0) = (4 m)ˆ and vectorv (0) = (2 m / s)ˆ ı . If its acceleration is constant and given by vectora = − (2 m / s 2 ) (ˆ ı +ˆ ), at what time t does the particle first cross the x axis? Correct answer: 2 s. Explanation: vectorr ( t ) = vectorr (0) + vectorv (0) t + 1 2 vectora t 2 , so vectorr ( t ) = (4 m) ˆ + (2 m / s) ˆ ı t +( − 1 m / s 2 ) (ˆ ı +ˆ ) t 2 = [(2 m / s) t − (1 m / s 2 ) t 2 ]ˆ ı +[4 m − (1 m / s 2 ) t 2 ] ˆ vectorr ( t ) will not have a y component when 4 m − (1 m / s 2 ) t 2 = 0 (1 m / s 2 ) t 2 = 4 m t = 2 s . 002 (part 2 of 2) 10.0 points At what time t is the particle moving parallel to the y axis; that is, in the − ˆ ı direction? Correct answer: 1 s. Explanation: vectorr ( t ) = vectorr (0) + vectorv (0) t + 1 2 vectora t 2 vectorv ( t ) = dvectorr d t = vectorv (0) + vectora t , so vectorv ( t ) = (2 m / s)ˆ ı − (2 m / s 2 ) (ˆ ı +ˆ ) t = [(2 m / s) − (2 m / s 2 ) t ]ˆ ı − (2 m / s 2 ) t ˆ vectorv ( t ) will not have an x component when 2 m / s − (2 m / s 2 ) t = 0 t = 1 s . At this time vectorv ( t ) = − (2 m / s)ˆ parallel to the yaxis. 003 10.0 points A home run is hit such a way that the baseball just clears a wall 20 m high located 146 m from home plate. The ball is hit at an angle of 39 ◦ to the horizontal, and air resistance is negligible. Assume the ball is hit at a height of 1 m above the ground.of 1 m above the ground....
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This document was uploaded on 10/26/2011 for the course PHYS 1710 at North Texas.
 Fall '08
 Weathers
 mechanics, Work

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