ME221Lecture 351 ME 221 Statics Lecture #35 Sections 8.1 – 8.3

ME221Lecture 352 Homework #12 Chapter 8 problems: –1, 2, 4, 8 & 15 Due Monday, December 1

ME221Lecture 353 No Class Wednesday, November 26

ME221Lecture 354 Quiz #8 Monday, December 1 Friction problem

ME221Lecture 355 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

ME221Lecture 356 Chapter 8 - Friction Overview Coulomb friction (dry friction) –Static and kinematic Fluid friction –Wedges –Types

ME221Lecture 357 Friction Angle A body on an incline, acted upon by gravity alone, will slip at an angle related to the coefficient of friction N f W  From the FBD: N – W cos α = 0 & ƒ – W sin α = 0 Such that: ƒ = N tan α

ME221Lecture 358 Friction Angle The Coefficient of Static Friction (  s ) is defined as the tangent of the maximum angle a body may be inclined before slip occurs N f W   s = tan  max = ƒ max / N The angle  max is called the friction angle

ME221Lecture 359 Coulomb Friction (dry friction) Friction force (ƒ) is proportional to the normal force (N) and opposes motion Coefficient of friction, , is the proportionality constant –Static coefficient,  s –Kinematic coefficient,  k kNkN sNsN P f P N f W f=P

ME221Lecture 3510 Common Static Friction Coefficients Steel on steel0.75 Rubber on concrete Rubber on ice Metal on wood Teflon on Teflon0.04 Tires on gravel0.5

ME221Lecture 3511 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

ME221Lecture 3512 Example Problems