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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}
Historically the first mathematical description of [interactions|interaction] in Newtonian Mechanics 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  Forces produce a change inexample, the earth and the moon are attracted by the motionforce of gravity, aOR massby ontheir which they act, according to F=ma ([gravitational interaction.


*Properties of forces:*

* Forces satisfy 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_ ([ (for a point particle) 
* The definition of force and its unit of measurement (Newton = kg m s-2) follow from this law
* Various forces combine linearly according to laws of _vector addition_ to give the _Net Force_ acting on a body.
* Interactions between bodies generate opposite forces on each of the two different bodies (Newton's Third Law]).{excerpt}
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* Four fundamental interactions give rise to the four fundamental forces, but only gravity and the electromagnetic forces are ficant at distance scales larger than a micron.
* _Phenomenological Forces_ such as contact forces, friction, spring force, tension, drag from a moving fluid, etc. result from these fundamental forces plus quantum mechanics and often follow mathematical laws reasonably accurately.  (e.g. Hooke's Law, viscous drag, form drag)
* Conservative forces can be expressed as the (negative) gradient of the associated potential energy.

h3. Motivation for Concept

Consider a bowling ball (or some other heavy object that moves with little resistance).  If you want thea 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]**:*  
{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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