## Fundamental Properties, Forces, and Laws

In classical physics, mass and electric charge are two fundamental properties of physical objects—that is, they are properties that are not explained in terms of simpler or more basic properties. (In contrast, color is a non-fundamental property, since it can be explained in terms of the wavelengths of light that an object reflects.) The standard unit of mass is the kilogram (kg), and the standard unit of charge is the coulomb (C).

Mass is not the same thing as weight. Mass is a measure of the amount of matter in an object, and is independent of location or gravity. Weight is the force of gravity exerted on an object, and varies depending on the object’s location. (Objects weigh less on the moon than they do on Earth, for example.)

Each of these fundamental properties is associated with a fundamental force—a force that isn’t explained in terms of more basic forces. (In contrast, the force of your pen pressing against a paper is not fundamental, since it can be explained in terms of more basic forces—namely, the electromagnetic repulsion between molecules.)  The gravitational force acts between any two objects that have mass; the electromagnetic force acts between any two objects that have charge.

The fundamental laws of classical physics describe the mathematical relations between fundamental properties and forces, but do not explain why these relations hold. In other words, the fundamental laws are not explained in terms of further regularities or causes; they are simply taken for granted as unexplained regularities in nature. (After formulating his law of universal gravitation, Newton famously admitted: “I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypotheses…”)This famous quote is from the General Scholium of Newton’s Principia.

The following table shows the two fundamental properties of classical physics, with their corresponding forces and laws:

Properties Forces Laws
mass gravity Newton’s law of universal gravitation
charge electromagnetism Coulomb’s law, the Lorentz force law, and Maxwell’s equations
We’ll examine each of these laws later in this chapter.