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- I. Describe the interactions this system experiences.
Interactions are influences that change the motion of the system. The most common way to describe interactions is to think of them as forces and represent them using a free body diagram, but later you will become familiar with energy and torque as alternate descriptions. There are only a few types of interactions that we will study in this course, including gravity, contact forces (pushes, pulls, friction, etc.) and spring forces. It is important to recognize that interactions always occur between two objects. For instance, if your system is subject to a gravitational interaction, it must be near another object (often the earth). If your system is subject to a contact force, there must be another object nearby to do the pushing or pulling.
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