Forces and Free Body Diagrams.
Newton’s concept of inertia includes a clause that allows the motion of objects to change, which is through the use of a force. Forces are understood as an influence (such as a push or a pull) that can cause an object to change its motion. Forces can have all sorts of sources. Forces can be caused by gravity, magnetism, or the physical interaction of objects. At this stage, we aren’t interested in what causes the forces. All we need to do is focus on the idea that they exist and they can cause changes to motion.
Forces are vectors. This means that they have a direction and a magnitude. So when we think about how a force might be interacting with an object, we have to pay attention to the direction of the force. This will tell us whether the force is causing the object to speed up, slow down, or change directions (or some combination of those). The direction of the force is the direction that the object would start to move if it was at rest.
When thinking about the forces that are being applied to an object, we often draw a picture known as a free body diagram. This is a picture that represents the object and all of the forces that are being applied to it. This helps us to organize the information in a problem. Since forces are vectors, they can be added together. The net force is the result of adding together all of the forces that are acting on an object. If an object is maintaining its motion, then the net force must be zero.
We will explore these diagrams in more detail in a later section.