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# Lecture6 - PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 6...

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PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 6

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Gravity Normal forces Strings, ropes and Pulleys Friction Work and Kinetic Energy Potential Energy Conservation of Energy Last Lecture: Today:
Frictional Forces RESISTIVE force between object and neighbors or the medium Examples: Sliding a box Air resistance Rolling resistance

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Sliding Friction Parallel to surface, opposing direction of motion ~ independent of the area of contact Depends on the surfaces in contact Object at rest: Static friction Object in motion: Kinetic friction
Static Friction, s Just enough force to keep object at rest. μ s is coefficient of static friction N is the normal force f F f s μ

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Kinetic Friction, k μ k is coefficient of kinetic friction Friction force opposes direction of motion N is the normal force F f f k = m k N μ k < m s
Coefficients of Friction f μ = μ μ μ

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Example 4.7 The man pushes/pulls with a force of 200 N. The child and sled combo has a mass of 30 kg and the coefficient of kinetic friction is 0.15. For each case:
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