chap8.pdf - Chapter 8 Friction What is friction Why do we...

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Chapter 8 - Friction
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What is friction?
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Why do we need friction?
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= a force of resistance between surfaces Friction always acts parallel (or tangential) to surfaces in contact Opposes the motion (or intended motion) of one surface over another Two types of friction between surfaces Dry friction – two dry surfaces in contact. ● Fluid friction – surfaces separated by film of fluid (gas or liquid) the latter not covered this course What is Friction?
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● An easy way to understand dry friction is to consider pulling horizontally on a block of weight W which is resting on a rough surface. ● How many forces are there on the block? 4 forces on block W – weight P – pulling force N – normal force F – friction ● 3 states for surface static equilibrium impending motion motion Dry Friction
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APPLICATIONS For an applied force on the bike tire brake pads, how can we determine the magnitude and direction of the resulting friction force? In designing a brake system for a bicycle, car, or any other vehicle, it is important to understand the frictional forces involved.
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APPLICATIONS What physical factors affect the answer to this question? The rope is used to tow the refrigerator. In order to move the refrigerator, is it best to pull up as shown, pull horizontally, or pull downwards on the rope?
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CHARACTERISTICS OF DRY FRICTION Friction is defined as a force of resistance acting on a body which prevents or resists the slipping of a 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.
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CHARACTERISTICS OF DRY FRICTION For the body shown in the figure to be in equilibrium, the following must be true: F = P, N = W, and W*x = P*h.
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CHARACTERISTICS OF DRY FRICTION For the body shown in the figure to be in equilibrium, the following must be true: F = P, N = W, and W*x = P*h . Recap: 5.3 Equations of Equilibrium if a rigid body is subjected to a system of forces which all lie in the x - y plane then the resulting moment about the z axis is the same for any arbitrary point O within the x - y plane. In short: For the problems considered in this chapter the origin O for the computation of the total moment can be chosen arbitrarily.
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CHARACTERISTICS OF DRY FRICTION 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).
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IMPENDING TIPPING versus SLIPPING If the friction force F is large the man needs to exert a large pushing force P that eventually will lead to tipping details of when this is happening will be discussed further below
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