Mass



For alternative meanings, see mass (disambiguation).
Mass is a property of physical objects which, roughly speaking, measure the amount of matter contained in an object. It is a central concept of classical mechanics and related subjects. In the SI system of measurement, mass is measured in kilograms.
Strictly speaking, there are two different quantities called mass:
  • Inertial mass is a measure of an object's inertia, which is its resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
  • Gravitational mass is a measure of the strength of an object's interaction with the gravitational force. Within the same gravitational field, an object with a smaller gravitational mass experiences a smaller force than an object with a larger gravitational mass. (This quantity is sometimes confused with weight.)
Inertial and gravitational mass have been experimentally shown to be equivalent, as accurately as we can measure, although they are conceptually quite distinct. This equivalence between inertial and gravitational mass is at the heart of the theory of general relativity. Below, we will discuss the definitions and implications of each of these two quantities.