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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 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 led you to choose the model you are using (for example, if there are no external interactions, a momentum model is a good choice).
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