Lecture18%20Friction%20and%20Drag%20Forces-handout

# Lecture18%20Friction%20and%20Drag%20Forces-handout - Today...

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1 Today Newton’s 3 rd Law continued Friction and air drag

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Newton’s Third Law Weight and normal force are not an action/reaction pair. Action/reaction partners have same type, act w n
W12 Q3 A 1200 kg car pushes a 2800 kg truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push against the ground with a force of 1500 N. What is the magnitude of the force of the truck on the car? Hint: Identify the forces and the correct action/reaction pair.

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Solution The blue arrows show the correct action/reaction pair: Force of the car on the truck (F ConT ) and the Force of the truck on the car (F TonC ). • The force exerted by the ground on the car F GonC is shown in red . We know it’s 1500 N. For the car, 2 nd law yields: F GonC – F = m C a • For the truck we obtain: F ConT = m T a • Substitute for acceleration a and solve for F (Ans.: 1050 N)
Tension Forces A student is pulling on a rope that is fixed to a tree. His force is 100 N. 1) What is the tension in the rope? 2) What is the force of the tree on the rope? A. 0 N B. 100 N C. 200 N D. More than 200 N

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Tension Forces Two students are pulling on a rope. Each students pulls with a force of 100 N. What is the tension in the rope? A. 0 N B. 100 N C. 200 N D. More than 200 N 100 N 100 N
Tension Forces Two students pulling on a rope vs. pulling on a rope fixed to a wall. a) Newton’s third law: F S on L = 100 W = T L on S Newton’s 2 nd law: T L on R – T R on L = 0 Newton’s third law: T R on W = T W on R = 100 N b) Rope does not move: Second student exerts same force on the rope as wall in situation (a)

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## This note was uploaded on 05/19/2011 for the course PHYS PHYS 100 taught by Professor Lioudmila during the Winter '10 term at UBC.

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Lecture18%20Friction%20and%20Drag%20Forces-handout - Today...

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