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

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1 Physics for Scientists & Engineers 1 February 3, 2011 Physics for Scientists&Engineers 1 1 Spring Semester 2011 Lecture 11 Friction
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2 Friction square4 Introduction of a new force: friction square4 Friction forces are present in practically all kinematical processes square4 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 February 3, 2011 Physics for Scientists&Engineers 1 2 ( 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 ) The friction force is independent of speed ( experiment 6 )
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3 Two Types of Friction square4 There are two types of friction Kinetic friction Object in motion Static friction Object at rest; static friction force has a maximum value square4 Both types of friction are proportional to the normal force February 3, 2011 Physics for Scientists&Engineers 1 3 square4 The coefficient μ is always greater than zero and usually smaller than 1 square4 The coefficient is different for kinetic friction and for static friction f = μ N
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4 Kinetic Friction square4 Kinetic friction deals with objects in motion square4 The magnitude of kinetic friction is given by square4 N is the magnitude of the normal force square4 μ k is the coefficient of kinetic friction square4 The direction of the kinetic friction force on an object is f k = μ k N February 3, 2011 Physics for Scientists&Engineers 1 4 always opposite to the direction of the motion of the object square4 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
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5 Static Friction square4 If an object is at rest, it takes a threshold amount of force to make it move square4 If you push on a stationary object with a force below the threshold, it will not move square4 If you push on the stationary object hard enough, it will begin to move February 3, 2011 Physics for Scientists&Engineers 1 5 square4 The static friction force is always equal to and opposite the force exerted on the stationary object square4 We can write the static friction force as f s μ s N = f s,max
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6 Tribology - Science of Friction - Science of Friction
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