# ch5 - Notes on Chapter 5 Friction Consider applying a small...

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John Ellison, UCR p.1 Notes on Chapter 5 Friction Consider applying a small horizontal force to a massive block placed on a tabletop (refer to the figure at right). Unless the force is greater than a certain critical force the box will not move. This is because a frictional force is directed in the opposite direction to the applied force with equal magnitude. The force is called the static frictional force . As the applied force is increased increases and the block remains at rest. However, once the applied force exceeds a critical value the block suddenly starts moving and accelerates in the direction of the applied force. The frictional force that then opposes the motion is called the kinetic frictional force , and is usually less than the maximum magnitude of the static frictional force. Therefore, once the block starts moving, you can keep it moving at constant velocity with less force than you needed to get it started. Experiments show that the frictional force arises from the attractive forces between atoms on the two surfaces in contact. On a microscopic scale the surfaces are rough and are "in contact" in many small regions which make up only a tiny fraction of the total surface area of the objects. Attractive forces between atoms in these contact regions result in a net frictional force. If a body is at rest on a surface, the force of static friction has whatever magnitude and direction are required to keep it at rest. The magnitude of has a maximum value given by where μ s is the coefficient of static friction and N is the magnitude of the normal force on the body from the surface. f s f s f s f k f s , max s N f s

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John Ellison, UCR p.2 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 μ k is the coefficient of kinetic friction . The normal force is a measure of how hard the object is pressing on the surface. The coefficients of friction μ s and μ k are dimensionless parameters that must be determined by experiment. They depend on the nature of the surfaces which are in contact. Example The figure at right shows a coin of mass m at rest on a book that has been tilted at an angle θ with the horizontal. By experimenting, you find that when θ is increased to 13°, the coin is on the verge of sliding down the book, which means that even a slight increase beyond 13° produces sliding. What is the coefficient of static friction μ s between the coin and the book? Solution
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ch5 - Notes on Chapter 5 Friction Consider applying a small...

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