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Internal interactions are interactions between two system constituents.

External interactions are interactions between a system constituent and the environment.

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.

Conservative vs. Non-Conservative

Conservative interactions are interactions for which a potential energy can be consistently defined.

Non-conservative interactions cannot be described by a potential energy.

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.

Specifying Interactions in a Solution

When specifying the interactions involved as part of a problem solution, it is permissible 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, only external interactions are relevant, since internal interactions 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 led you to choose the model you are using (for example, if there are no external interactions, a momentum model is a good choice).