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Single-Axis Rotation of a Rigid Body

Description and Assumptions

This model is applicable to a single rigid body that is rotating about a fixed axis or else both rotating and translating in such a way that its angular momentum 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 Angular Momentum and External Torque about a Single Axis 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 the fixed axis or its center of mass.

Problem Cues

This model is useful for a stationary object (the special case of statics). In that case, both the linear acceleration a and the angular acceleration α are zero, and there is the additional freedom that the axis 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.

Learning Objectives

Students will be assumed to understand this model who can:

Model

Compatible Systems

One rigid body whose moment of inertia calculated about the chosen axis of rotation is constant.

Relevant Interactions

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 axis of rotation. Allowed choices of the axis of rotation will depend upon the specific
circumstances of the problem:

Laws of Change

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Diagrammatic Representations

Relevant Examples

ExamplesInvolvingaFixedAxis"> Examples Involving a Fixed Axis

ExamplesInvolvingaRotatingandTranslatingRigidBody"> Examples Involving a Rotating and Translating Rigid Body

ExamplesInvolvingaStaticRigidBody"> Examples Involving a Static Rigid Body

AllExamplesUsingthisModel"> All Examples Using this Model



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Photos courtesy:
Wikimedia Commons by Johann H. Addicks
U.S. Navy by Mass Communications Specialist 3rd Class Walter M. Wayman

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