Lecture 10

# Lecture 10 - LECTURE 10 FRICTION Contents INTRODUCTION...

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1 LECTURE 10 FRICTION Contents INTRODUCTION CHARACTERISTICS OF COULOMB FRICTION

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2 I NTRODUCTION Thus far, the forces between two surfaces in contact have been assumed either smooth or perfectly rough. A rough surface is the surface that can provide whatever force to ensure equilibrium. Friction forces are to oppose the tendency of contacting surfaces to slip relative to one another and can be good or bad. Sometimes, we want to maximize the fraction in brakes and belt drivers. In many cases, friction is not desirable since it can cause energy loss and wear down sliding surfaces in contact. Tow major types of friction are commonly encountered in engineering practice: dry friction or c oulomb friction describes the tangential component of the contact force that exists when two dry surfaces slide or tend to slide relative to one another fluid friction is the tangential component of the contact force that exists between adjacent layers in a fluid that are moving at different velocities relative to each other. C HARACTERISTICS OF COULOMB FRICTION RESULTANT FORCE FROM GROUND For the block of mass, at resting on a rough horizontal surface and acted on by a horizontal force P , as shown in Fig. 9-1 a . The resultant force R from the ground has the following characteristics:
3 It passes through the interacting point of W and P for the equilibrium of the block, as shown in Fig. 9-1 b . The resultant force R can be represented by a normal component, N , and a horizontal or friction component, F , on the contact surface. As P increases from zero, F also increases. F will be sufficient to permit the line of action of the reaction R to reach the corner A at the block. IMPENDING MOTION The maximum value that F can reach is called the limit value of static friction , beyond which, the block will move under the action of P . The condition when the friction force is at its maximum value is called the condition of impending motion .

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Lecture 10 - LECTURE 10 FRICTION Contents INTRODUCTION...

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