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When one object has the potential to alter the exerts a force that may change the state of motion (translational or rotational) of another object, those objects are said to interact. An object that interacts with another experiences an interaction. There are only a handful of types of physical interactions, for example gravity, contact, electrostatic, etc., and each one always causes opposite forces on each of the two interacting bodies. |
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There are many ways that one object can change the motion of another. A person may kick a ball across the ground, giving it a translational motion, or instead may spin a ball on their finger, giving it rotational motion. The earth changes the motion of objects through the conservative action-at-a-distance of gravity (what goes up must come down) as well as its electrostatic charge, and also through the nonconservative interaction of friction (what goes out will need some help getting back)air resistance. Introductory physics incorporates several ways of describing interactions to capture some of the richness of the world.
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Important
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Consequences of Interactions in Introductory Physics
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Internal interactions are interactions between two system constituentsobjects, both constituentsof the system under consideration.
External interactions are interactions between a system constituent and an object in the environment, that is one outside the system.
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.
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When specifying the interactions involved as part of a problem solution, it is permissible necessary 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 will lead you to choose the appropriate model you are using (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. |