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Lecture05_2011 - PHYSICS 220 Lecture 05 Forces and Motion...

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Lecture 5 Purdue University, Physics 220 1 Lecture 05 Forces and Motion beyond 1 D Textbook Sections 3.7, 4.1 PHYSICS 220
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Lecture 5 Purdue University, Physics 220 2 mg N y x Apparent Weight Recall: Σ F = m a Consider person accelerating up in an elevator Draw FBD Apply Newton’s second law N – mg = ma N = m(g+a) Apparent weight is equal to normal force from scale or floor Note: in free fall a=-g so N=0
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Lecture 5 Purdue University, Physics 220 3 Σ F = ma N – mg = ma N = m(g+a) a < 0 so N < mg mg N iClicker You are traveling up on an elevator to the 30 th floor of the Sears tower. As it nears the 30 th floor, your weight appears to be A) heavier B) the same C) lighter
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Lecture 5 Purdue University, Physics 220 4 A person has a mass of 50 kg. What is their apparent weight when they are riding on an elevator Going up with constant speed 9.8 m/s Going down with constant speed 9.8 m/s Accelerating up at a rate of 9.8 m/s 2 Accelerating down at a rate of 9.8 m/s 2 N = m(g+a) a = 0 so N= mg = 490 Newtons a = 0 so N= mg = 490 Newtons a = +9.8 m/s 2 so N= 2 mg = 980 Newtons a = -9.8 m/s 2 so N= 0 mg = 0 Newtons Exercise
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Lecture 5 Purdue University, Physics 220 5 y x A B C A B C Exercise You are standing on a scale inside an elevator. You weigh 125 pounds, but the scale reads 140 pounds.
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