PHY183-Lecture13pre - Friction Introduction of a new force:...

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1 February 2, 2012 Physics for Scientists&Engineers 1 1 Physics for Scientists & Engineers 1 Spring Semester 2012 Lecture 13 Friction February 2, 2012 Physics for Scientists&Engineers 1 2 Friction Introduction of a new force: friction Friction forces are present in practically all kinematical processes 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 ( experiment 1 ) If an object is at rest, then the force one must exert to start it moving is larger than the force required to keep it moving ( experiment 2 ) The friction force is proportional to the normal force ( experiment 3 ) The friction force is independent of the size of the contact area ( experiment 4 ) The friction force depends on the roughness of the contact surface ( experiment 5 – we’ll get to this shortly ) The friction force is independent of speed February 2, 2012 Physics for Scientists&Engineers 1 3 Two Types of Friction There are two types of friction Kinetic friction • Object in motion Static friction • Object at rest; static friction force has a maximum value Both types of friction are proportional to the normal force The coefficient μ is always greater than zero and usually smaller than 1 The coefficient is different for kinetic friction and for static friction f = μ N February 2, 2012 Physics for Scientists&Engineers 1 4 Kinetic Friction Kinetic friction deals with objects in motion The magnitude of kinetic friction is given by N is the magnitude of the normal force μ k is the coefficient of kinetic friction The direction of the kinetic friction force on an object is always opposite to the direction of the motion of the object 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? Only two forces are acting, friction force and pushing force Newton’s First Law: net force must be zero, because object moves with constant velocity => friction force in this case is exactly opposite to the pushing force f k = k N February 2, 2012 Physics for Scientists&Engineers 1 5 Static Friction If an object is at rest, it takes a threshold amount of force to make it move
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This note was uploaded on 04/02/2012 for the course PHY 183 taught by Professor Wolf during the Spring '08 term at Michigan State University.

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PHY183-Lecture13pre - Friction Introduction of a new force:...

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