Mass is an
intrinsic property of a body. It was traditionally believed to be related to the
quantity of
matter in a
physical body, until the discovery of the
atom and
particle physics. It was found that different atoms and different
elementary particles, theoretically with the same amount of matter, have nonetheless different masses. Mass in modern physics has multiple
definitions which are conceptually distinct, but physically
equivalent. Mass can be experimentally defined as a
measure
Measure may refer to:
* Measurement, the assignment of a number to a characteristic of an object or event
Law
* Ballot measure, proposed legislation in the United States
* Church of England Measure, legislation of the Church of England
* Mea ...
of the body's
inertia, meaning the resistance to
acceleration (change of
velocity) when a
net force is applied. The object's mass also determines the
strength of its
gravitation
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
al attraction to other bodies.
The
SI base unit
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all ...
of mass is the
kilogram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...
(kg). In
physics, mass is
not the same as
weight, even though mass is often determined by measuring the object's weight using a
spring scale
A spring scale, spring balance or newton meter is a type of mechanical force gauge or weighing scale. It consists of a spring fixed at one end with a hook to attach an object at the other. It works in accordance with Hooke's Law, which states th ...
, rather than
balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Phenomena
There are several distinct phenomena that can be used to measure mass. Although some theorists have speculated that some of these phenomena could be independent of each other, current experiments have found no difference in results regardless of how it is measured:
* ''Inertial mass'' measures an object's resistance to being accelerated by a force (represented by the relationship
).
* ''Active gravitational mass'' determines the strength of the gravitational field generated by an object.
* ''Passive gravitational mass'' measures the gravitational force exerted on an object in a known gravitational field.
The mass of an object determines its acceleration in the presence of an applied force. The inertia and the inertial mass describe this property of physical bodies at the qualitative and quantitative level respectively. According to
Newton's second law of motion
Newton's laws of motion are three basic laws of classical mechanics that describe the relationship between the motion of an object and the forces acting on it. These laws can be paraphrased as follows:
# A body remains at rest, or in motion ...
, if a body of fixed mass ''m'' is subjected to a single force ''F'', its acceleration ''a'' is given by ''F''/''m''. A body's mass also determines the degree to which it generates and is affected by a
gravitational field
In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
. If a first body of mass ''m''
A is placed at a distance ''r'' (center of mass to center of mass) from a second body of mass ''m''
B, each body is subject to an attractive force , where is the "universal
gravitational constant
The gravitational constant (also known as the universal gravitational constant, the Newtonian constant of gravitation, or the Cavendish gravitational constant), denoted by the capital letter , is an empirical physical constant involved in ...
". This is sometimes referred to as gravitational mass.
[When a distinction is necessary, the active and passive gravitational masses may be distinguished.] Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical; since 1915, this observation has been incorporated ''
a priori'' in the
equivalence principle
In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (suc ...
of
general relativity.
Units of mass
The
International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...
(SI) unit of mass is the
kilogram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...
(kg). The kilogram is 1000 grams (g), and was first defined in 1795 as the mass of one cubic decimetre of water at the
melting point of ice. However, because precise measurement of a cubic decimetre of water at the specified temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of a metal object, and thus became independent of the metre and the properties of water, this being a copper prototype of the
grave in 1793, the platinum
Kilogramme des Archives in 1799, and the platinum-iridium
International Prototype of the Kilogram
The International Prototype of the Kilogram (referred to by metrologists as the IPK or Le Grand K; sometimes called the '' ur-kilogram,'' or ''urkilogram,'' particularly by German-language authors writing in English) is an object that was used t ...
(IPK) in 1889.
However, the mass of the IPK and its national copies have been found to drift over time. The
re-definition of the kilogram and several other units came into effect on 20 May 2019, following a final vote by the
CGPM
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
in November 2018. The new definition uses only invariant quantities of nature: the
speed of light, the
caesium hyperfine frequency, the
Planck constant and the
elementary charge
The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
.
[
]
Non-SI units accepted for use with SI units include:
* the
tonne (t) (or "metric ton"), equal to 1000 kg
* the
electronvolt (eV), a unit of
energy, used to express mass in units of eV/''c''
2 through
mass–energy equivalence
In physics, mass–energy equivalence is the relationship between mass and energy in a system's rest frame, where the two quantities differ only by a multiplicative constant and the units of measurement. The principle is described by the physicis ...
* the
dalton (Da), equal to 1/12 of the mass of a free
carbon-12
Carbon-12 (12C) is the most abundant of the two stable isotopes of carbon (carbon-13 being the other), amounting to 98.93% of element carbon on Earth; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 i ...
atom, approximately .
[The dalton is convenient for expressing the masses of atoms and molecules.]
Outside the SI system, other units of mass include:
* the
slug
Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a smal ...
(sl), an
Imperial unit of mass (about 14.6 kg)
* the
pound
Pound or Pounds may refer to:
Units
* Pound (currency), a unit of currency
* Pound sterling, the official currency of the United Kingdom
* Pound (mass), a unit of mass
* Pound (force), a unit of force
* Rail pound, in rail profile
Symbols
* Po ...
(lb), a unit of mass (about 0.45 kg), which is used alongside the similarly named
pound (force) (about 4.5 N), a unit of force
[These are used mainly in the United States except in scientific contexts where SI units are usually used instead.]
* the
Planck mass (about ), a quantity derived from fundamental constants
* the
solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
(), defined as the mass of the
Sun, primarily used in astronomy to compare large masses such as stars or galaxies (≈ )
* the mass of a particle, as identified with its inverse
Compton wavelength ()
* the mass of a star or
black hole
A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
, as identified with its
Schwarzschild radius ().
Definitions
In
physical science, one may distinguish conceptually between at least seven different aspects of ''mass'', or seven physical notions that involve the concept of ''mass''.
Every experiment to date has shown these seven values to be
proportional
Proportionality, proportion or proportional may refer to:
Mathematics
* Proportionality (mathematics), the property of two variables being in a multiplicative relation to a constant
* Ratio, of one quantity to another, especially of a part compare ...
, and in some cases equal, and this proportionality gives rise to the abstract concept of mass. There are a number of ways mass can be measured or
operationally defined:
* Inertial mass is a measure of an object's resistance to acceleration when a
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
is applied. It is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says the body of greater mass has greater
inertia.
* Active gravitational mass
[The distinction between "active" and "passive" gravitational mass does not exist in the Newtonian view of gravity as found in ]