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When one object exerts a force that may change the state of motion (translational or rotational) of another object, those objects are said to interact. |
Motivation for Concept
There are many ways that one object can change the motion of another. A person may kick a ball across the ground, giving it a translational motion, or instead may spin a ball on their finger, giving it rotational motion. The earth changes the motion of objects through the conservative
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action-at-a-distance
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of
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gravity
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as
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well
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as
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its
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electrostatic
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charge,
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and
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also
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through
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the
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nonconservative
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interaction
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of
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air
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resistance.
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Introductory
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physics
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incorporates
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several
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ways
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of
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describing
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interactions.
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Common
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Interactions
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Several
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specific
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interactions
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are
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commonly
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encountered
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in
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mechanics.
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Describing Interactions
Physicists have developed many ways to describe the effects of interactions. Each different description can be applied to any of the specific types of interactions listed above, with the exception that only a conservative interaction can be consistently described as a potential energy.
- Acceleration:
Excerpt Include RELATE:acceleration RELATE:acceleration nopanel true - Force:
Excerpt Include RELATE:force RELATE:force nopanel true - Impulse:
Excerpt Include RELATE:impulse RELATE:impulse nopanel true - Work:
Excerpt Include RELATE:work RELATE:work nopanel true - Potential Energy:
Excerpt Include RELATE:potential energy RELATE:potential energy nopanel true - Torque:
Excerpt Include RELATE:torque (single-axis) RELATE:torque (single-axis) nopanel true - Angular Impulse:
Excerpt Include RELATE:angular impulse RELATE:angular impulse nopanel true
Classifying Interactions
Internal vs. External
For both linear and angular momentum models, interactions that take place between two system constituents will cancel from the Law of Change as a result of Newton's 3rd Law. Thus, when using a momentum or angular momentum model, it is important to classify the interactions as internal or external:
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Conservative vs. Non-Conservative
For energy models, conservative interactions should be represented by their associated potential energy, while non-conservative interactions must be accounted for as work. Thus, when using an energy model, it is important to classify the interactions as conservative or non-conservative.
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Torque-Producing vs. Non-Torque-Producing
For angular momentum models, forces whose line of action pass directly through the chosen axis of rotation have no effect on the rotational motion of the system. Thus, when using such a model, it is important to classify the interactions as torque-producing or non-torque-producing.
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For this category, the appropriate classification will depend upon your choice of rotation axis. Sometimes making a careful choice of axis can reduce the number of torque-producing (and hence relevant) forces in a problem. |
Specifying Interactions in a Solution
When specifying the interactions involved as part of a problem solution, it is only necessary to focus on the interactions which are relevant to the model that you will be using. For example, if a momentum model is being used to describe the motion of a system consisting of more than one object, only external interactions are relevant, since internal interactions between the object in the system will cancel from the Law of Change as a result of Newton's 3rd Law. When you are specifying the interactions, you should indicate the characteristics that will lead you to choose the appropriate model (for example, if there are no external interactions, a momentum model is a good choice).
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As this statement implies, it is impossible to clearly specify the relevant interactions for a given problem without having the system and a model in mind. {children:page=Interaction Glossary|excerpt=true|all=true} h3. Describing Interactions Physicists have developed many ways to describe the effects of interactions. Each different description can be applied to _any_ of the specific types of interactions listed above, with the exception that only a [conservative interaction|conservative force] can be consistently described as a [potential energy]. * [*Acceleration*|acceleration]: {excerpt-include:acceleration|nopanel=true} * [*Force*|force]: {excerpt-include:force|nopanel=true} * [*Impulse*|impulse]: {excerpt-include:impulse|nopanel=true} * [*Work*|work]: {excerpt-include:work|nopanel=true} * [*Potential Energy*|potential energy]: {excerpt-include:potential energy|nopanel=true} * [*Torque*|torque (single-axis)]: {excerpt-include:torque (single-axis)|nopanel=true} * [*Angular Impulse*|angular impulse]: {excerpt-include:angular impulse|nopanel=true} h3. Classifying Interactions h4. Internal vs. External For both [linear|Momentum and External Force] and [angular momentum|Angular Momentum and External Torque about a Single Axis] [models|model], interactions that take place between two [system constituents|system constituent] will cancel from the Law of Change as a result of [Newton's 3rd Law|Newton's Third Law]. Thus, when using a momentum or angular momentum model, it is important to classify the interactions as internal or 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 For [energy|Mechanical Energy and Non-Conservative Work] models, conservative interactions should be represented by their associated potential energy, while non-conservative interactions must be accounted for as [work]. Thus, when using an energy model, it is important to classify the interactions as conservative or non-conservative. [*Conservative Force*|conservative force]: {excerpt-include:conservative force|nopanel=true} [*Non-Conservative Force*|non-conservative force]: {excerpt-include:non-conservative force|nopanel=true} h4. Torque-Producing vs. Non-Torque-Producing For [angular momentum|Angular Momentum and External Torque about a Single Axis] models, forces whose [line of action] pass directly through the chosen [axis of rotation] have no effect on the rotational motion of the [system]. Thus, when using such a model, it is important to classify the interactions as torque-producing or non-torque-producing. {note}For this category, the appropriate classification will depend upon your choice of [rotation axis|axis of rotation]. Sometimes making a careful choice of axis can reduce the number of torque-producing (and hence relevant) forces in a problem.{note} h3. Specifying Interactions in a Solution When specifying the interactions involved as part of a problem solution, it is only necessary to focus on the interactions which are _relevant_ to the [model] that you will be using. For example, if a [momentum|Momentum and External Force] model is being used to describe the motion of a [system] consisting of more than one object, only [_external_ interactions|external force] are relevant, since [internal interactions|internal force] between the object in the system will cancel from the [Law of Change] as a result of [Newton's 3rd Law|Newton's Third Law]. When you are specifying the interactions, you should indicate the characteristics that will lead you to choose the appropriate model (for example, if there are no external interactions, a momentum model is a good choice). {note}As this statement implies, it is impossible to clearly specify the relevant interactions for a given problem without having the [system] and a [model] in mind. {note} {td} {tr} {table} {live-template:RELATE license} |