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Friction is a force that opposes motion.
Static friction is the frictional force between two objects that are NOT moving relative to each other. It is the initial force that must be overcome in order for objects to move. If an object is stationary, and the force trying to move the object is less than the maximum possible force of static friction, the object will not move.
Kinetic friction is the frictional force between two surfaces that ARE moving relative to each other (sliding along each other). Once an object has overcome static friction, it has kinetic friction acting on it.
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In the diagram to the right, you can see the opposing relationship between applied force and friction. As the applied force increases, the opposing frictional force also increases until the mass starts moving. This is a static frictional force. When the applied force reaches the maximum static friction, the mass begins to move; after the mass begins moving, kinetic friction acts upon it. Static friction is greater than kinetic friction, so once the mass begins sliding, it takes less force to keep it sliding.
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You can duplicate both types of friction by placing your hands together and pushing them against each other. Start to move them in a sliding motion. The motion is resisted by the texture of your skin and the magnitude of the applied force. This is static friction. Now that they are moving relative to each other, kinetic friction comes into play.
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There are two factors which determine the maximum frictional force that can occur between two surfaces: coefficient of friction and normal force.
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The maximum force of friction (Ff) between two surfaces is equal to the coefficient of friction (Cf) of those two surfaces multiplied by the normal force (N) holding those surfaces together. Ff = Cf x N
See Also
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