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h1. Contact Force

{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}

h3. 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.  

h3. Special Cases

Because contact forces are so common in mechanics, there are several important special cases of contact forces that are commonly encountered.

h4.  [Applied Force

|applied force]: {excerpt-include:applied force}  When a person purposefully presses against another object, the resulting contact force is often called an applied force.

h4. [Collision Forces|collision forces]: {excerpt-include:collision forces}

h4. [Friction|friction]: {excerpt-include:friction}

h4.  [Normal Force|normal force]: {excerpt-include: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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