PHY183-Lecture11 - 1 Physics for Scientists & Physics...

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Unformatted text preview: 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 February 3, 2011 Physics for Scientists&Engineers 1 1 Spring Semester 2011 Lecture 11 Friction 2 Friction Friction Friction Friction s Introduction of a new force: friction s Friction forces are present in practically all kinematical processes s Concerning friction we observe the following • If an object is at rest, then it takes a threshold of an external force to make it move ( e x p e r i m e n t 1 e x p e r i m e n t 1 ) • If an object is at rest, then the force one must exert to start it February 3, 2011 Physics for Scientists&Engineers 1 2 moving is larger than the force required to keep it moving ( e x p e r i m e n t 2 e x p e r i m e n t 2 ) • The friction force is proportional to the normal force ( e x p e r i m e n t 3 e x p e r i m e n t 3 ) • The friction force is independent of the size of the contact area ( e x p e r i m e n t 4 e x p e r i m e n t 4 ) • The friction force depends on the roughness of the contact surface ( e x p e r i m e n t 5 e x p e r i m e n t 5 ) • The friction force is independent of speed ( e x p e r i m e n t 6 e x p e r i m e n t 6 ) 3 Two Types of Friction Two Types of Friction Two Types of Friction Two Types of Friction s There are two types of friction • Kinetic friction • Object in motion • Static friction • Object at rest; static friction force has a maximum value s Both types of friction are proportional to the normal force February 3, 2011 Physics for Scientists&Engineers 1 3 s The coefficient μ is always greater than zero and usually smaller than 1 s The coefficient is different for kinetic friction and for static friction f = μ N 4 Kinetic Friction Kinetic Friction Kinetic Friction Kinetic Friction s Kinetic friction deals with objects in motion s The magnitude of kinetic friction is given by s N is the magnitude of the normal force s μ k is the coefficient of kinetic friction he direction of the kinetic friction force on an object is f k = μ k N February 3, 2011 Physics for Scientists&Engineers 1 4 s The direction of the kinetic friction force on an object is always opposite to the direction of the motion of the object s If we push on an object to keep it sliding at a constant speed, the magnitude of the friction force is equal to the magnitude of the force with which we are pushing. Why?...
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This note was uploaded on 04/23/2011 for the course PHY 183 taught by Professor Wolf during the Spring '08 term at Michigan State University.

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PHY183-Lecture11 - 1 Physics for Scientists & Physics...

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