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Section 6.1 Newton’s First Law of Motion

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The Greeks viewed the world as being made of four fundamental elements, and each of these elements had a "natural state" where those elements wanted to be. For example, rocks fell because they wanted to return to the earth. However, Newton’s First Law of Motion rejected such philosophical musings and replaced them with with a concrete rule that applied to all objects and all situations.

The Idea of Newton’s First Law: Inertia.

Definition 6.1.0.1.

An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by a net force.
The basic idea is simple. Objects don’t just move arbitrarily. They move because something caused them to move. And if they are in motion, they will maintain that motion until something causes the motion to change. The tendency of objects to resist changing their motion (or lack of motion) is called inertia. The amount of inertia an object has is known as its mass. It’s very important to remember that mass and weight are not the same concept. Even in the weightlessness of outer space, objects will still have mass and will still resist your attempts to change their motion.

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.