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ME 221 Statics Lecture #35 Sections 8.1 – 8.3

ME 221 Statics Lecture #35 Sections 8.1 – 8.3. Homework #12. Chapter 8 problems: 1, 2, 4, 8 & 15 Due Monday, December 1. No Class Wednesday, November 26. Quiz #8. Monday, December 1 Friction problem. Final Exam. Section 1 (3:00pm) Monday, 12/8, 3:00 – 5:00pm Section 2 (12:40pm)

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ME 221 Statics Lecture #35 Sections 8.1 – 8.3

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  1. ME 221 StaticsLecture #35Sections 8.1 – 8.3 Lecture 35

  2. Homework #12 Chapter 8 problems: • 1, 2, 4, 8 & 15 Due Monday, December 1 Lecture 35

  3. No ClassWednesday, November 26 Lecture 35

  4. Quiz #8 Monday, December 1 Friction problem Lecture 35

  5. Final Exam Section 1 (3:00pm) Monday, 12/8, 3:00 – 5:00pm Section 2 (12:40pm) Wednesday, 12/10, 12:45 – 2:45pm See University policy for multiple exams Lecture 35

  6. Chapter 8 - Friction • Overview • Types • Coulomb friction (dry friction) • Static and kinematic • Fluid friction • Wedges Lecture 35

  7. Friction Angle • A body on an incline, acted upon by gravity alone, will slip at an angle related to the coefficient of friction • From the FBD: • N – W cos α = 0 & • ƒ – W sin α = 0 • Such that: • ƒ = N tan α W f N  Lecture 35

  8. Friction Angle • The Coefficient of Static Friction (ms) is defined as the tangent of the maximum angle a body may be inclined before slip occurs ms = tan max =ƒmax / N W • The angle max is called the friction angle f N  Lecture 35

  9. Coulomb Friction (dry friction) • Friction force (ƒ) is proportional to the normal force (N) and opposes motion • Coefficient of friction, m, is the proportionality constant f • Static coefficient, ms msN • Kinematic coefficient, mk mkN W f=P P f P N Lecture 35

  10. Common Static Friction Coefficients Steel on steel 0.75 Rubber on concrete 0.5-0.9 Rubber on ice 0.05-0.30 Metal on wood 0.2-0.6 Teflon on Teflon 0.04 Tires on gravel 0.5 Lecture 35

  11. Friction: Wedges • Wedges are a simple means for lifting • Again, friction force opposes the motion • Disassembled FBDs are essential for solving wedge problems • Apply equilibrium equations to disassembled FBDs Lecture 35

  12. Example Problems Lecture 35

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