Chapter_6 - FRICTION CHAPTER 6 BMFG 1823 Static and...

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Unformatted text preview: FRICTION CHAPTER 6 BMFG 1823 Static and Strength of Material STATIC & STRENGTH OF MATERIALS FRICTION (Sections 8.1 - 8.2) Today’s Objective: Students will be able to: a) Understand the characteristics of dry friction. b) Draw a FBD including friction. c) Solve problems involving friction. STATIC & STRENGTH OF MATERIALS READING QUIZ 1. A friction force always acts _____ to the contact surface. A) normal B) at 45° C) parallel D) at the angle of static friction 2. If a block is stationary, then the friction force acting on it is ________ . A)   s N B) =  s N C)   s N D) =  k N STATIC & STRENGTH OF MATERIALS APPLICATIONS In designing a brake system for a bicycle, car, or any other vehicle, it is important to understand the frictional forces involved. For an applied force on the brake pads, how can we determine the magnitude and direction of the resulting friction force? STATIC & STRENGTH OF MATERIALS APPLICATIONS (continued) Consider pushing a box as shown here. How can you determine if it will slide, tilt, or stay in static equilibrium? What physical factors affect the answer to this question? STATIC & STRENGTH OF MATERIALS CHARACTERISTICS OF DRY FRICTION (Section 8.1) Friction is defined as a force of resistance acting on a body which prevents or retards slipping of the body relative to a second body. Experiments show that frictional forces act tangent (parallel) to the contacting surface in a direction opposing the relative motion or tendency for motion. For the body shown in the figure to be in equilibrium, the following must be true: F = P, N = W, and Wx = Ph. STATIC & STRENGTH OF MATERIALS CHARACTERISTICS OF FRICTION (continued) To study the characteristics of the friction force F, let us assume that tipping does not occur (i.e., “h” is small or “a” is large). Then we gradually increase the magnitude of the force P. Typically, experiments show that the friction force F varies with P, as shown in the left figure above. STATIC & STRENGTH OF MATERIALS FRICTION CHARACERISTICS (continued) The maximum friction force is attained just before the block begins to move (a situation that is called “impending motion”). The value of the force is found using F s =  s N , where  s is called the coefficient of static friction. The value of  s depends on the materials in contact. Once the block begins to move, the frictional force typically drops and is given by F k =  k N. The value of  k (coefficient of kinetic friction) is less than  s . STATIC & STRENGTH OF MATERIALS DETERMING  s EXPERIMENTALLY A block with weight w is placed on an inclined plane. The plane is slowly tilted until the block just begins to slip....
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This document was uploaded on 10/26/2011 for the course FKP bmfp at UTEM Chile.

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Chapter_6 - FRICTION CHAPTER 6 BMFG 1823 Static and...

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