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{excerpt}A [force] that arises when one macroscopic body presses against another.  When one of the bodies is a flat surface
like a floor, wall or ramp the contact force is usually split into components called the [friction] force and
the [normal force].{excerpt}

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h2. Motivation for Concept

When one macroscopic body encounters another, the bodies cannot simply pass through each other.  Instead, their contact results in a change in the motion of each body.  

h2. Special Cases

h4.  Applied Force

When a person purposefully presses against another object, the resulting contact force is often called an applied force.

h4.  Normal Force

An object in contact with a surface will always be subject to a contact force that has a component perpendicular to the surface.  That perpendicular component guarantees that the object will not pass through the surface.  It receives the special name of [normal force], derived from the mathematical meaning of normal (perpendicular).

h4. Friction

An object in contact with a surface will sometimes experience a contact force that has a component parallel to the surface.  Since the _perpendicular_ component receives a special label as the normal force, the parallel component is also generally given a name:  the force of [friction].  This division of the contact force is useful, since in many cases the [friction] component of the contact force can be related to the [normal|normal force] component.  

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