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{excerpt:hidden=true}*[System|system]:* One [rigid body] rotating about a fixed axis or rotating and translating such that its angular momentum is constrained to one-dimension and its moment of inertia is constant. --- *[Interactions|interaction]:* Any that respect the [one-dimensional angular momentum|angular momentum (one-dimensional)].{excerpt}


h1. Single-Axis Rotation of a Rigid Body

h4. {toggle-cloak:id=desc} Description and Assumptions

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This model is applicable to a _single_ [rigid body|rigid body] that is rotating about a fixed axis or else both rotating and translating in such a way that its [angular momentum|angular momentum (one-dimensional)] is a one-dimensional vector (usually taken to lie along the z-axis). As a special case, this model is also useful to constrain the forces acting on a static rigid body.  It is a subclass of the [1-D Angular Momentum and Torque] model defined by the constraint that the system consists of only one rigid body which has a fixed mass and a fixed moment of inertia for rotations about its center of mass.
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h4. {toggle-cloak:id=cues} Problem Cues

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This [model|model] is useful for a stationary object (the special case of _statics_).  In that case, both the linear [acceleration|acceleration] _a_ and the angular acceleration α are zero, and there is the additional freedom that the [axis|axis of rotation] can be placed at any point in the object.  For accelerating objects, the model is commonly used in cases where a single object is placed in a situation where the forces are well understood, such as a cylinder rolling down an inclined plane or a sphere rolling along level ground.  Often, the linear and angular accelerations will be related by the [rolling without slipping] condition.     
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h4. {toggle-cloak:id=pri}Prior Models

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* [Point Particle Dynamics]

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h4. {toggle-cloak:id=vocab} Vocabulary

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* [force]
* [free body diagram]
* [torque (one-dimensional)]

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h2. Model

h4. {toggle-cloak:id=sys} {color:red}Compatible Systems{color}

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One [rigid body|rigid body].

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h4. {toggle-cloak:id=int} {color:red} Relevant Interactions {color}

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Only external torques need be considered, as internal torques do not produce angular accelerations.  Since torque is needed, forces must be specified not only by their magnitude and direction, but also by either their point of application or [moment arm] with respect to the center of mass of the rigid body.

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h4. {toggle-cloak:id=laws} {color:red} Laws of Change {color}

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{latex}\begin{large} \[ \sum \tau_{a} = I_{a}\alpha\]\end{large}{latex}

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h4. {toggle-cloak:id=diag} {color:red} Diagrammatic Representations {color}

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h2. Relevant Examples


h4. {toggle-cloak:id=fix} Examples Involving a Fixed Axis

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h4. {toggle-cloak:id=trans} Examples Involving a Rotating and Translating Rigid Body

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h4. {toggle-cloak:id=stat} Examples Involving a Static Rigid Body

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h4. {toggle-cloak:id=all} All Examples Using this Model

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!pisa.jpg|width=235px!
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Photos courtesy:
[Wikimedia Commons|http://commons.wikimedia.org/wiki/File:Pisa_-_Campo_Santo_-_Campanile_4_-_2005-08-08_17-30_4832.JPG] by [Johann H. Addicks|mailto:addicks@gmx.net]
[U.S. Navy|http://www.navy.mil] by Mass Communications Specialist 3rd Class Walter M. Wayman
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