PHYS171_f11_exam_3_example_solution

# PHYS171_f11_exam_3_e - PHYS171 Exam 3 practice Name Fall 2011 Student ID Short Answer Questions 1 You need to launch an object at sufficient

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PHYS171 Exam 3 practice Fall 2011 Name: _____________________ Student ID #: ________ 1 Short Answer Questions 1) You need to launch an object at sufficient velocity so that it entirely escapes the earth’s gravitation field (i.e., at escape velocity). To minimize the energy required it is better to (a) launch from the highest mountain accessible (b) launch from the deepest valley accessible (c) it doesn’t matter, why? (5 points) You must generate kinetic energy to propel you to a higher potential energy state. If you start from a higher altitude your potential is higher to begin with, so a smaller increase in potential energy is necessary. Answer is (a) 2) You have a slingshot that launches an object at a fixed velocity. Standing on the roof of this building, you must use it to launch your iPhone so that it lands on the ground below, at which point you can assume it comes instantly to rest. To minimize the risk of damage, should you (a) launch it straight up (b) launch it straight down (c) launch it horizontally (d) it does not matter. Why? (5 points) If damage depends only on change in velocity it does not matter what direction the phone is thrown. After falling a height h the kinetic energy increases by mgh , so the final speed is the same independent of the initial direction. 3) A baseball bat is 1 meter long, and weighs 0.75 kg. What is the effective moment of intertia when a baseball player swings the bat (10 points)? The bat can be approximated as a thin rod, with angular momentum ML 2 /12 around the center of mass. The bat is swung around its end, so we should correct this using the parallel axis theorem, which gives ML 2 /3. The moment of inertia is therefore (0.75 kg)(1 m) 2 /3 = 0.25 kg m 2 . 4) One cylinder rolls down an inclined plane without slipping and a second identical cylinder slides down the same inclined plane without friction. Which cylinder’s center of mass is moving faster when it reaches the bottom of the inclined plane. (a) rolling of (b) sliding.

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## This note was uploaded on 12/29/2011 for the course PHYSICS 171 taught by Professor La porta during the Fall '11 term at Maryland.

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PHYS171_f11_exam_3_e - PHYS171 Exam 3 practice Name Fall 2011 Student ID Short Answer Questions 1 You need to launch an object at sufficient

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