Up until now, we have focused on describing the motion of an object without worrying about the factors that cause the object to move the way that is does.  If we want our discussion to include questions about why an object’s motion is what it is, then we have crossed over from kinematics into dynamics. Dynamics is the study of how forces affect the way that objects move.

To begin with, we need a definition for force.  Like many of the other key topics in physics such as energy, work, and power, we use the word “force” all the time in everyday conversation.  However, the colloquial way we use the word force may be inconsistent with the physics definition for the word.  The most basic physics definition, the one that arguably makes the most sense to us intuitively, is that forces are how objects push or pull on each other.  Fundamentally, forces are the way that two objects interact.  We will begin by talking about forces that arise from interactions between solids, such as normal forces, friction forces, tension forces, and elastic forces.  We will also include some of the forces that result from the interaction between a solid and a liquid, like buoyancy, lift, thrust, and drag.  There are also a set of forces that are due to one object interacting with the field created by another object.  We will discuss the gravitational force this semester and leave the electromagnetic forces for the second semester of the course.

In working dynamics problems, we will start by identifying the forces that act on an object of a system of objects.  Forces are vectors, meaning they have both a magnitude and a direction, and part of the process of identifying a force will be recognizing in which direction the force must act.  We will draw free-body diagrams which show all the forces that act on an object, giving us a visual representation of the interaction an object has with the other objects around it.  Identifying the forces and drawing them into a free-body diagram is the critical first step in solving almost any problem using Newton’s Three Laws of Motion. This module should give us the practice we need to be comfortable creating free-body diagrams so we can use them in the next module to solve problems using Newton’s laws.