Newton's First Law

First, a little history:

The ancients thought that the natural state of any object was at rest and that, in order for something to move, it needed to have a force applied to it. Galileo realized this wasn't accurate, but it was Sir Isaac Newton's work that quantified the concepts of force and how they affect objects through three very important laws. The first of these is The Law of Inertia. Newton's First Law states that

An object at rest tends to remain at rest, and an object in motion continues in motion with
a constant velocity, unless the object is acted upon by a net external force.

So what is this inertia thing? Inertia is a property of matter and it is one way we measure an object's mass. Inertia is a resistance to motion. Inertia is why it is hard to start a car rolling by pushing it, and also why it is hard to stop it once you get it going! The more massive an object is, the more it tends to resist a change in motion.

When an object is at rest or moving at a constant velocity, it is said to be in equilibrium. There is no net external force acting on it. In other words, when we add up all of the force vectors, the resultant is zero.

This brings us to a quick side discussion of mass. Newton defined mass as a "quantity of matter" or how much matter is in an object. Today, mass is defined in terms of the inertia of an object, or how hard it is to cause a change in an object's motion. Obviously, physical size does not matter - very large objects can have very little mass, such as a giant styrofoam ball, while very small objects can be incredibly dense and have very large mass. Mass, in the SI system, is measured in kilograms. Frequently, the term weight is confused with mass. Weight is a term used to describe the force of gravity acting on an object. Weight is a force, given as the quantity of mass times the acceleration on an object due to local gravity. An objects mass will remain constant (unless we add or take away matter) but an objects weight changes depending on its location. For example, an object has a different weight on the moon than on the earth. The mass is the same in both locations, but since gravity is different, the weight is different.

Often it is helpful to see what someone else has to say on the topic. Try

http://www.physicsclassroom.com/Class/newtlaws/U2L1a.cfm

http://www.physicsclassroom.com/Class/newtlaws/U2L1b.cfm

http://www.physicsclassroom.com/Class/newtlaws/U2L1c.cfm

http://www.physicsclassroom.com/Class/newtlaws/U2L1d.cfm

for more information.