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ENGI 1313 Mechanics I

ENGI 1313 Mechanics I . Lecture 36: Friction. Chapter 8 Objectives. to introduce the concept of dry friction to analyze the equilibrium of rigid bodies subjected to dry friction force to present specific applications of dry friction force analysis on wedges. Lecture 36 Objective.

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ENGI 1313 Mechanics I

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  1. ENGI 1313 Mechanics I Lecture 36: Friction

  2. Chapter 8 Objectives • to introduce the concept of dry friction • to analyze the equilibrium of rigid bodies subjected to dry friction force • to present specific applications of dry friction force analysis on wedges

  3. Lecture 36 Objective • to introduce the concept of dry friction • to analyze the equilibrium of rigid bodies subjected to dry friction force

  4. Friction – What is it? • Resistance force • Sliding, rolling, twisting • Tangent to normal contact surface between two bodies • Acts in a direction opposite to relative motion or tendency for motion

  5. Types of Friction • Fluid Friction • Contact surface separated by fluid • Fluid may be a liquid or gas • Fluid mechanics

  6. Types of Friction • Dry Friction • Coulomb friction • Non-lubricated contact surface

  7. Dry Friction • Classical Model • Mechanical • Surface roughness • Other Factors • More complex process • Scale dependent • Macro  nano scale • Parameters with varying importance • e.g. compare friction between two ‘rough’ sheet metal and two polished sheet metal surfaces • e.g. compare friction between two rough glass and two smooth glass (microscope plates) surfaces

  8. Dry Friction – Classical Model • Mechanical • Surface roughness Frictional component, F Normal Force, N

  9. Dry Friction – Classical Model (cont.) • Distributed Contact Forces • Normal • Tangential (Frictional) • Resultant Forces

  10. F F   N N N Normal Force, N

  11. Static Friction Force, Fs • Equilibrium (P < Fs) Why is Fs N? s  coefficient of static friction

  12. Static Friction Force, Fs (cont.) • Impending Motion (P = Fs = sN) Impending Motion Fs Friction Force P = Fs No Motion Applied Force

  13. Friction Force • Assumptions • Proportional to normal force • Independent of the contact area • Independent of velocity • Valid for a Wide Range of Practical Conditions

  14. Friction Force (cont.) • Can Breakdown • Contact conditions • Variable friction coefficient e.g. Tires in snow or mud

  15. Static Friction Force, Fk Constant Velocity • Motion (P > Fk = kN) • Block is in motion • Constant velocity • Kinetic friction coefficient ~25% less than static •  Fk < Fs • Complex phenomenon k  coefficient of kinetic friction where k < s

  16. Variation of Friction Force • Three Phases • Static friction • Equilibrium • Limiting static friction • Maximum value • Impending motion • Kinetic friction • Motion Impending Motion Fs Kinetic Static Friction Force P = Fs No Motion Applied Force

  17. Variation of Friction Force (cont.) • Three Characteristics • Experimental measurements • Range where static friction exceeds kinetic friction • General decrease in friction force magnitude and coefficient • General constant kinetic friction force Fs Kinetic Static Friction Force P = Fs No Motion Applied Force

  18. Determination of Friction Coefficient Exercise: Sum forces on axes parallel and perpendicular to plane Static: Angle to cause motion Kinetic: Angle to cause motion with constant velocity

  19. Impending Motion / Tipping? • How to Determine? • Number of Unknowns? • Fs, N, P, and x

  20. Impending Motion / Tipping? (cont.) • Assume Impending Motion • Slipping • What is Known or Assumed? • What is to be Solved? • P, N, x • Check?

  21. Impending Motion / Tipping? (cont.) • Assume Tipping • What is Known? • What is to be Solved? • P, N, F • Check?

  22. Comprehension Quiz 21-01 • A 10 N block is in equilibrium. What is the magnitude of the friction force between this block and the surface? • A) 0 N • B) 1 N • C) 2 N • D) 3 N • Answer: C

  23. Textbook Problems

  24. Textbook Problems

  25. References • Hibbeler (2007) • http://wps.prenhall.com/esm_hibbeler_engmech_1

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