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h1. Force
{excerpt}Historically, the first mathematical description of [interactions|interaction] in Newtonian Mechanics. , and still the most commonly used.  In this WIKI, "force" is often used interchangeably with "interaction".  Forces produce a change in the motion of a mass on which they act, according to F=ma ([Newton's Second Law]), which is a vector equation (the acceleration is in the direction of the net force).   Forces result from several types of physical interactions, which always exert _opposite_ forces on two _different objects_ ([Newton's Third Law]).{excerpt}
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h3. Motivation for Concept

Consider a bowling ball (or some other heavy object that moves with little resistance).  If you want the ball to move, you have to exert a force on it in the direction you want it to move.  If you want the moving ball to turn, you have to exert a force on it toward the side you want it to turn toward.  If you want the ball to stop moving, you have to exert a force opposite to its velocity.  To change the motion of the bowling ball, you will probably apply a force by using your hands or feet or some object you push against the ball.  There are other kinds of forces, however.  The earth, for example, can alter the ball's motion through the invisible action-at-a-distance of [gravity|gravity (near-earth)], often represented as a gravitational field acting on the body at the site of the body.

h3. Newton's Laws

Newton's famous Three Laws of Motion together comprise his definition of force.

* *[Newton's First Law]:* {excerpt-include:Newton's First Law|nopanel=true}
* *[Newton's Second Law]:* {excerpt-include:Newton's Second Law|nopanel=true}
* *[Newton's Third Law]:* {excerpt-include:Newton's Third Law|nopanel=true}

h3. Classification of Forces 

There are many ways to classify forces.  For the purposes of the modeling approach to physics, the most important classifications to understand are Internal vs. External and Conservative vs. Non-Conservative.   Another commonly encountered classification of forces is by their status as "fundamental" vs. phenomenological.

h4. Internal vs. External

* *[Internal Force|internal force]:*  {excerpt-include:internal force|nopanel=true}

* *[External Force|external force]:*  {excerpt-include:external force|nopanel=true}

h4. Conservative vs. Non-Conservative

* *[Conservative Force|conservative force]:* {excerpt-include:conservative force|nopanel=true}

* *[Non-Conservative Forces|non-conservative force]:* {excerpt-include:non-conservative force|nopanel=true}

h4. Fundamental vs. Phenomenological

* *[Fundamental Forces|fundamental forces]:* {excerpt-include:fundamental forces|nopanel=true}

* *[Phenomenological Forces|phenomenological forces]:* {excerpt-include:phenomenological force|nopanel=true}


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