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The Friction Force

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Push your hand across your desktop and feel the force called

friction opposing the motion

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Friction is often minimized in solving force and motion

problems, but in the real world, friction is everywhere. You need it to both start and stop a bike and a

car.

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If you’ve ever walked on ice, you know how important friction is.

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Friction lets a pencil make a mark on paper and an eraser fix

mistakes.

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Think about friction as you push a heavy crate across the floor.

You give the crate a push, but it doesn’t move.

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Newton’s laws tell you a crate should move unless there is a second horizontal force on the crate, opposite in direction to your force, and equal in size.

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That force is called the static friction force.

(It is exerted on one surface by the other when there is no relative motion between the two surfaces.)

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You can push harder and harder but if the crate still doesn’t move, the friction force must also be getting larger. The static friction force acts in response to other forces.

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Finally, when you push hard enough, the crate begins to move. Evidently, the static friction force

can grow only so large.

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The crate may be moving, but friction is still acting because if

you stop pushing, the crate slows. The force that is acting is called

the kinetic friction force.

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The kinetic friction force is the force exerted on one surface by

the other when the surfaces are in relative motion.

(This is also know as sliding friction.)

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The magnitude of the friction force is proportional to the

magnitude of the force pushing one surface against the other.

That force, perpendicular to the surface, is the normal force, FN.

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Kinetic friction force

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• In the equation,

• is a proportionality constant called the kinetic coefficient of friction.

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The static friction force is related to the normal force by this

equation.

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Where is the static coefficient of friction.

This equation tells you that the static friction force can vary from

zero to where is the maximum static force that must be balanced before motion

can begin.