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h1. Force
{excerpt} Force produces a change in the momentum of a mass on which it acts, according to F=ma ([Newton's Second Law]). Forces result from various types of physical interactions, which always generate a pair of _opposite_ forces acting on two _different objects_ ([Newton's Third Law]).{excerpt}
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Historically the first mathematical description of [interactions|interaction] was by forces and force laws, and this formulation is still the most commonly used in Newtonian Mechanics. In the traditional approach to Newtonian Mechanics, all other descriptions of interactions (e.g. potential energy) are _defined_ in terms of force. In this WIKI, "force" is often used interchangeably with "interaction". For example, the earth and the moon are attracted by the force of gravity, OR by their gravitational interaction.

h3. Motivation for Concept

Consider a bowling ball (or some other heavy object that moves with little resistance).  If you want a stationary ball to move, you have to exert a force on it in the direction you want it to move, which will accelerate it.  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]**:*
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* *[*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 forces|nopanel=true}
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