# Friction - Physics 201 Lecture 7 Friction What is it How do...

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3/27/08 Physics 201, UW-Madison 1 Physics 201: Lecture Physics 201: Lecture 7 Friction What is it? How do we characterize it? Model of friction Some problems involving friction Drag Forces Terminal speed Exam -- Monday Feb. 18 At 5:45pm in 3650 Humanities Topics: Chapters 1-4

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3/27/08 Physics 201, UW-Madison 2 Surface Friction. .. Surface Friction. .. Friction is caused by “microscopic” interactions between two surfaces:
3/27/08 Physics 201, UW-Madison 3 Question 1 Question 1 with different surface areas A A > A B are placed on the floor. The force needed to set box A in motion compared to that on box B is: larger the same smaller A B

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3/27/08 Physics 201, UW-Madison 4 Question 2 Question 2 A = A B and material but with different masses, m A < m B are placed on the floor. The force needed to set box A in motion compared to that on box B is: larger the same smaller A B
3/27/08 Physics 201, UW-Madison 5 Model for Surface Friction Model for Surface Friction The direction of the frictional force vector f F is perpendicular to the normal force vector N , in the direction opposing relative motion of the two surfaces. Kinetic (sliding): Kinetic (sliding): The magnitude of the frictional force vector is proportional to the magnitude of the normal force N . f F = μ K N Static: Static: The frictional force balances the net applied forces such that the object doesn’t move. The maximum possible static frictional force is proportional to N . f F μ S N The “heavier” something is, the greater the friction will be. ..makes sense! Not dependant on the area of contact!! These relations are all useful APPROXIMATIONS to messy reality.

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3/27/08 Physics 201, UW-Madison 6 Surface Friction. .. Surface Friction. .. Force of friction acts to oppose relative motion: Parallel to surface. Perpendicular to N ormal force. m a F f F m g N i j j
3/27/08 Physics 201, UW-Madison 7 Friction Friction What does it do? It is in opposition to relative motion How do we characterize this in terms we have learned? Friction results in a force in the direction opposite to the direction of relative motion! m a F APPLIED f FRICTION m g N i j j

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3/27/08 Physics 201, UW-Madison 8 Model. .. Model. .. Dynamics: j : N = mg i : F μ K N = ma so F μ K mg = ma m a F m g N i j j μ K mg
3/27/08 Physics 201, UW-Madison 9 Problem Problem 1: Friction 1: Friction A box of mass m 1 = 1.5 kg is being pulled by a horizontal string having tension T = 90 N . It slides with friction ( μ k = 0.51 ) on top of a second box having mass m 2 = 3 kg , which in turn slides on a frictionless floor. What is the acceleration of the second box ?

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## This note was uploaded on 03/27/2008 for the course PHYS 201 taught by Professor Walker during the Spring '08 term at University of Wisconsin.

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Friction - Physics 201 Lecture 7 Friction What is it How do...

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