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Classifyinig Interactions
Internal vs. External
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, 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
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).