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10_05_Friction - 10/5 Friction(Kinetic 0 0 0 0 Text Chapter...

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10/5 Friction (Kinetic) 0 Text: Chapter 4 section 9 friction 0 Problem presentation 0 HW 10/4 “Block up a Ramp” due Wed 10/10 0 Suggested Problems: Ch 4: 56, 58, 60, 74, 75, 76, 79, 102 0 For Monday, check out chapter 5 (briefly) as you lab is about circular motion.
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N B,A Newton’s Laws Example The elevator is moving down and slowing down. Draw a free body diagram for each box. Find all the forces on both boxes as they come to rest from 2m/s in a time of 0.25s. B A a = B A W E,B N A,B W E,A a = v/ t = 2/.25 = 8m/s 2 N T,A F net,A = = F net,B m A = 9kg, m B = 10kg. F net = ma W = mg F net,A = 9(8) = 72N F net,B = 10(8) = 80N W E,A = 9(10) = 90N W E,B = 10(10) = 100N F net,B = 80N = N A,B - W E,B 100N N A,B = 180N So N B,A = 180N F net,A = 72N = N T,A - (W E,A + N B,A ) 270N N T,A = 342N
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Newton’s Laws Example Now the elevator starts from rest and heads back up, speeding up to 2m/s in 0.25 s. Draw a free body diagram for each box. Find all the forces on both boxes during this t. B A a = a = v/ t = 2/.25 = 8m/s 2 F net,A = = F net,B m A = 9kg, m B = 10kg. F net = ma N B,A B A W E,B N A,B W E,A N T,A W = mg F net,A = 9(8) = 72N F net,B = 10(8) = 80N W E,A = 9(10) = 90N W E,B = 10(10) = 100N F net,B = 80N = N A,B - W E,B 100N N A,B = 180N So N B,A = 180N F net,A = 72N = N T,A - (W E,A + N B,A ) 270N N T,A = 342N
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Newton’s Laws Example Now the elevator continues up at a constant a velocity of 2m/s.
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