interaction

When one object exerts a force that may change the state of motion (translational or rotational) of another object, those objects are said to interact.  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.

Motivation for Concept

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 as well as its electrostatic charge, and also through the nonconservative interaction of air resistance. Introductory physics incorporates several ways of describing interactions.

Important Consequences of Interactions in Introductory Physics

• force
• potential energy
• non-conservative work
• impulse
• torque
• angular impulse

Classifying Interactions

Internal vs. External

Internal interactions are interactions between two objects, 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.

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