MassMass is an intrinsic physical property of all objects which quantifies the way such objects will interact with other objects, fields of force, etc.. Mass, unlike weight, stays the same regardless of the environment surrounding the object. This is one of the central concepts behind classical mechanics and one of the main components of the theory of relativity.
There are three different ways of looking at mass. Inertial mass measures an object's resistance to a change in motion by a force. This is a critical part of Sir Isaac Newton's theories of mechanics. Mass is proportional to the amount of force required to affect it.
Passive gravitational mass determines an object's interaction with a gravity field. The quantity of this interaction is often referred to as an object's weight. Whereas an object's weight is relative to the strength of the gravitational field, its actual mass is unchanged. Weight and mass are often confused: to help remember the difference, keep in mind that weight is relative (your weight is different on the top of mountain peak as opposed to in a valley) but mass is constant (your mass would be the same at any point in time irregardless of where you were).
Active gravitational mass measures an object's direct gravitational impact on other objects. Because the Moon has less active gravitational mass, its gravity affects objects less than Earth's larger gravitational mass would.
Almost every physical theory assumes that these forms of mass are due to the same intrinsic property, and that the terms are simply different ways to describe their properties. It follows that mass and gravity are two inextricably entwined properties.
Web Resources On Mass
Mass and Weight
Weight vs. Mass; What's the Difference?
Book Resources On MassConcepts of Mass in Contemporary Physics and Philosophy by Max Jammer
Concepts of Mass in Classical and Modern Physics by Max Jammer
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