In Newtonian mechanics, mass and energy are two separate but related quantities. In a closed system, however, the total sum of mass and energy remain constant. To give the concept of energy a solid footing, it is best to consider mass first.

Often, mass is given a rather imprecise definition as the "amount of stuff" in an object. Galileo laid the foundation for a more rigorous idea of mass with his principle of inertia. The principle states that an object moving at a constant velocity in a straight line will continue moving at a constant velocity in a straight line as long as it is not disturbed, and an object sitting still will remain sitting still.

In his laws of motion, Isaac Newton linked inertia to mass. Newton's first law is essentially Galileo's principle of inertia, but with an important addition. The first law...