ME221Lecture 341 ME 221 Statics Lecture #34 Sections 8.1 – 8.2

ME221Lecture 342 Homework #11 Chapter 6 problems: –2, 3, 6 & 7 – Method of Joints –32, 36, 47 & 53 – Method of Sections –68 & 75 Due Today

ME221Lecture 343 Homework #12 Chapter 8 problems: –1, 2, 4, 8 & 15 Due Monday, December 1

ME221Lecture 344 Quiz #7 Today Analysis of Structures Method of Joints or Method of Sections

ME221Lecture 345 No Class Wednesday, November 26

ME221Lecture 346 No Class Friday, December 5 Mechanical Engineering Design Conference MSU Union 9am to noon Demonstrations 1pm to 2pm Awards in Ballroom ME371 Mechanical Design I ME412 Heat Transfer Lab ME456 Mechatronics Systems Lab ME471 Mechanical Design II ME481 Mech Engineering Design Projects

ME221Lecture 347 Chapter 8 - Friction Overview Coulomb friction (dry friction) –Static and kinematic Fluid friction –Wedges –Types

ME221Lecture 348 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 (friction force)

ME221Lecture 349 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 3410 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 3411 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 3412 Example Problem

ME221Lecture 3413 Quiz #7

ME221Lecture 3414 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 Examples: 8.71, 8.72, 8.80, 8.45