Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It may be seen as a statement about Inertia, that objects will remain in their state of motion unless a force acts to change the motion. Any change in motion involves an acceleration, and then Newton's Second Law applies; in fact, the First Law is just a special case of the Second Law for which the net external force is zero.

Mass commonly refers to any of three properties of matter, which have been shown experimenatally to be equivalent: Inertial Mass, Active gravitational mass and Passive gravitational mass. In everyday usage, mass is often taken to mean Weight, but in scientific use, they refer to different properties.

The inertial mass of an object determines its acceleration in the presence of an applied force. According to Isaac Newton's second law of motion, if a body of mass m is subjected to a force F, its acceleration a is given by F/m.

Inertia is the resistance of any physical object to a change in its state of motion. It is represented numerically by an object's mass. The principle of inertia is one of the fundamental principles of classical Physics which are used to describe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, "iners", meaning idle, or lazy. Sir Isaac Newton defined inertia in Definition 3 of his Philosophiæ Naturalis Principia Mathematica, which states:

The vis insita, or innate force of matter is a power of resisting, by which every body, as much as in it lies, endeavors to preserve in its present state, whether it be of rest, or of moving uniformly forward in a straight line.

In common usage, however, people may also use the term "inertia" to refer to an object's "amount of resistance to change in Velocity" (which is quantified by its mass), or sometimes to its momentum, depending on the context (e.g. "this object has a lot of inertia"). The term "inertia" is more properly understood as shorthand for "the principle of inertia" as described by Newton in his First Law of Motion. This law, expressed simply, says that an object that is not subject to any net external force moves at a constant velocity. In even simpler terms, inertia means that an object will always continue moving at its current speed and in its current direction until some force causes its speed or direction to change. This would include an object that is not in motion (velocity = zero), which will remain at rest until some force causes it to move.

Newton was once quoted for saying "If I have seen further it is only by standing on the shoulders of giants." Newton is responding to all of the acclaim that was showered upon him for he's laws of motion. Newton's giant is Galileo Galilei Galileo Galilei who's three laws on inertia is Newton's firsts law of motion.