cmch6 - 1 Chapter 6 Force and Motion - II In response to an...

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Unformatted text preview: 1 Chapter 6 Force and Motion - II In response to an applied force, a frictional force is directed in the opposite direction, exactly balancing the applied force. The frictional force is called the static frictional force. The block does not move. If you increase the magnitude of your applied force, the magnitude of the static frictional force also increases and the block remains at rest. s f s f 2 When the applied force reaches a certain magnitude, however, the block breaks away from its intimate contact with the tabletop and accelerates in the direction of the applied force. The frictional force that then opposes the motion is called the kinetic frictional force . The magnitude of the kinetic frictional force is usually less than the maximum magnitude of the static frictional force. k f 3 When two ordinary surfaces are placed together, only the high points touch each other. The actual micro scopic area of contact is much less than the apparent macro scopic contact area, perhaps by a factor of 10 4 . Nonetheless, many contact points do cold-weld together. These welds produce static friction when an applied force attempts to slide the surfaces relative to each other. 4 If the applied force is great enough to pull one surface across the other, there is first a tearing of welds (at breakaway) and then a continuous re-forming and tearing apart of welds as movement occurs and chance contacts are made. The kinetic frictional force that opposes the motion is the vector sum of the forces at those many chance contacts. If the two surfaces are pressed together harder, many more points cold-weld. Then, getting the surfaces to slide relative to each other requires a greater applied force: The static frictional force has a greater maximum value. s f k f 5 Properties of Friction 1. If the body does not move, then the static frictional force and the component of the applied force parallel to the surface balance each other. 2. The magnitude of the static frictional force has a maximum value given by where is the coefficient of static friction and N is the magnitude of the normal force on the body from the surface. If the magnitude of the component of the applied force parallel to the surface exceeds f s ,max , then the body begins to slide along the surface. 1. If the body begins to slide along the surface, the magnitude of the frictional force rapidly decreases to a value f k given by where is the coefficient of kinetic friction. Thereafter, during the s f s N f s max , s = N f k k = k 6 The magnitude N of the normal force increases if the...
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cmch6 - 1 Chapter 6 Force and Motion - II In response to an...

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