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Several specific interactions are commonly encountered in mechanics.

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• Acceleration:

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  RELATE:accelerationRELATE: acceleration nopanel true

• Force:

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• Impulse:

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  RELATE:impulseRELATE: impulse nopanel true

• Work:

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• Potential Energy:

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• Torque:

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• Angular Impulse:

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Internal Force:

External Force:

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.

Conservative Force:

Non-Conservative Force:

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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 on 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).