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ME 242 Exam 1 Solutions

# ME 242 Exam 1 Solutions - Name_KEY Last First UNLV...

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Name:______ KEY _________ Last First UNLV, DEPARTMENT OF MECHANICAL ENGINEERING MEG 242, Spring 2009, FIRST TEST Closed Book, one page of handwritten notes allowed. Enter the answer for each question into the space provided. Enter SI units in all answer spaces with brackets ( ). 1. (15 points) A ball is thrown vertically upright with a velocity of 30 m/s at the edge of a 50m cliff. Neglect air resistance. Determine: a) the height to which the ball rises. b) the time to reach the bottom of the cliff. Answers (a) v=0 at max. height, thus v*dv = a*dy Æ h = v 0 2 /2g = 45.87 m insert t into (b) time to top: vo – g*t = 0 Æ t1 = vo/g = 3.06 s Drop from top height to bottom is s = (45.87+ 50)m = ½*g*t 2 2 t2 = (2*95.87/g) 1/2 = 4.42 s Version B h = v 0 2 /2g = 20.38 m t1 = 20/g = 2.04 s Drop from top height to bottom is s = (20.38+ 40)m = ½*g*t 2 2 t2 = 3.51 s Answers: a) Max. height = 45.87 m Version B hmax = 20.38 m (8 pts) b) t bottom = 7.48 Version B t bottom = 5.55 s (7pts) 2. (10 points) A hockey puck sliding on a horizontal surface has a deceleration proportional to its speed a(v) = -0.5v in m/s 2 The initial velocity is v 0 = 50 m/s.

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