Mass (from Greek μάζα) is a concept used in the physical sciences to explain a number of observable behaviours, and in everyday usage, it is common to identify mass with those resulting behaviors. In particular, mass is commonly identified with weight. But according to our modern scientific understanding, the weight of an object results from the interaction of its mass with a gravitational field, so while mass is part of the explanation of weight, it is not the complete explanation.
For example, a mail carrier lifting a heavy package on earth may associate the heaviness (weight) of the package with the mass of its contents. This is a reasonable association for objects on earth. However, if the same package were on the moon it would weigh much less and would be easy to lift. Therefore, the mass of a package is only part of the reason that the package is difficult to lift on earth. The complete reason involves the interaction of the package’s mass with the gravity of the earth.
Also, a groundskeeper encountering two large rocks may associate the size of the rocks with their respective masses. And from this association the groundskeeper may expect the larger rock to be heavier and more difficult to move. However, if the larger rock were composed of pumice and the smaller of granite, then the smaller rock may in fact be much heavier. Mass is part of the explanation of an object’s size but not the complete explanation. The complete explanation involves mass, structure, and composition.
The human body is equipped with physical senses through which one can experience many of the effects associated with mass. One can visually observe an object to determine its size, lift it to feel its weight, and push it to feel the force of its inertial resistance to changing motion. These human experiences are all part of our modern understanding of mass, but none completely epitomizes the abstract concept of mass. The abstract concept did not come from a specific type of human experience. Rather, it came from a synthesis of many different types of human experience.
The modern concept was introduced in, and is central to, Isaac Newton’s explanation of gravitation and inertia. Prior to Newton’s time, the various gravitational and inertial phenomena were viewed as distinct and potentially unrelated. However, Isaac Newton united these phenomena by asserting that they all stemmed from a single underlying property called mass. Since Newton’s time, this abstract concept of mass has grown to include explanations for both quantum and relativistic effects. (See the following section entitled “Summary of concepts of mass” for a brief summary of mass-related phenomena.)
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