Static Frictional Forces Consider the example of two blocks, one resting on top of the other. If friction is present, a certain minimum horizontal force is required to move the top block. If a horizontal force less than this minimum force is applied to the top block, a force must act to counter the applied force and keep the block at rest. This force is called the static frictional force, and it varies according to the amount of force applied to the block. If no force is applied, clearly there is no static frictional force. As more force is applied, the static frictional force increases until it reaches a certain maximum value; once the horizontal force exceeds the maximum frictional force the block begins to move. The frictional force, defined as F s max , is conveniently proportional to the normal force between the two surfaces: F s max = μ s F N The constant of proportionality, μ s is called the coefficient of static friction, and is a property of the materials that are interacting (i.e. two interacting rough materials will have a higher value of
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