Physics with Calculus/Mechanics/Newton's First Law
Newton's first law states: In an inertial reference frame, unless acted on by a net force, an object at rest stays at rest. This is often referred to as inertia.
First Law (Inertia) 
Inertia is based on a simple concept: an object in motion tends to remain in motion. An object at rest tends to remain at rest. This is counter-intuitive to us, since we know that if we slide an object along the floor, it will slow down. Similarly, something may decide to fall off a shelf in a way that looks like it decided "all by itself". But Newton understood the basis behind this physical reality. When you slide a book along the floor it slows down, but not because of, as was sometimes thought, some property of matter that makes objects want to slow down. The book slows down because it interacts with the floor. Although this may seem a trivial difference from the way most people think about motion, it is really quite significant.
The concept of inertia refers to the tendency of an object to remain in motion, or to remain at rest. By intuition we can see that the tendency is somehow related to mass. It is easy to catch and stop a baseball; I don't recommend attempting to catch and stop a freight train. It's also a lot harder to move a loaded filing cabinet than it is to slide a single book. In order to measure how hard it was to overcome an object's inertia, Newton discovered another concept: