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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).
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As this statement implies, it is impossible to clearly specify the relevant interactions for a given problem without having the system and a model in mind. |