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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 action-at-a-distance of gravity as well as its electrostatic charge, and also through the nonconservative interaction of air resistance. Introductory physics incorporates several ways of describing interactions.

Important Consequences of Interactions in Introductory Physics

• force
• potential energy
• non-conservative work
• impulse
• torque
• angular impulse

Classifying Interactions

Internal vs. External

Internal interactions are interactions between two objects, both constituentsof the system under consideration.

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Common Interactions

Several specific interactions are commonly encountered in mechanics.

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:

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• Force:

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• Impulse:

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• Work:

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• Potential Energy:

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• Torque:

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• Angular Impulse:

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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.:

Internal Force:

External Force:

Conservative vs. Non-ConservativeConservative

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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.

Conservative Force:

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Non-Conservative Force:

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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.

Specifying Interactions in a Solution

When specifying the interactions involved as part of a problem solution, it is only necessary to focus only on the interactions which are relevant to the model that you will be using. For example, if a momentum model is being used on 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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