Mass
Mass measures inertia and gravitational attraction.
Mass describes how much a body resists being sped up or slowed down. In modern physics, it also measures how strongly an object pulls on other objects through gravity, as seen by an observer moving at the same speed. The standard unit for mass is the kilogram (kg). Since 1905, mass has also been understood as a measure of a body’s energy content, expressed by the equation E = mc².
Experimentally, mass can be defined as inertia—the resistance to a change in velocity when a force is applied. Mass is not the same as weight, though weight is often used to estimate mass. Weight is a force that depends on gravity; an object on the Moon weighs less than on Earth but keeps the same mass. In the Standard Model, the mass of elementary particles comes from their interaction with the Higgs boson, a process called the Brout–Englert–Higgs mechanism.
Several phenomena measure mass, and experiments show they give the same result. Inertial mass measures resistance to acceleration (F = ma). Active gravitational mass determines the strength of the gravitational field an object creates. Passive gravitational mass measures the gravitational force an object feels in a known field. Newton’s second law says a fixed mass m accelerates as a = F/m under a single force F. Gravitational mass appears in the equation Fg = GmAmB/r², where G is the universal gravitational constant. Since the 17th century, experiments have shown inertial and gravitational mass are identical, a fact built into general relativity’s equivalence principle since 1915.
The SI unit of mass, the kilogram, is now defined using the Planck constant, the speed of light, and the definition of the second. This replaced the older definition based on the platinum–iridium International Prototype of the Kilogram. Other widely used units include the tonne (1000 kg), the dalton (about 1.66×10⁻²⁷ kg, convenient for atoms and molecules), the electronvolt (used in high-energy physics as GeV/c²), the pound (about 0.45 kg), the Planck mass (about 2.18×10⁻⁸ kg), and the solar mass (about 1.99×10³⁰ kg, used in astronomy).
The kilogram was first defined in 1795 as the mass of one cubic decimetre of water at the melting point of ice. Because precise measurement was hard, it was redefined in 1889 as the mass of a metal object—first a copper prototype in 1793, then the platinum Kilogramme des Archives in 1799, and finally the platinum–iridium IPK in 1889. The IPK and its copies drifted over time, so on 20 May 2019 the kilogram was redefined using only invariant natural constants: the speed of light, the caesium hyperfine frequency, the Planck constant, and the elementary charge.
In general relativity, mass creates the gravitational field. Newtonian gravity distinguishes active, passive, and inertial mass, but Newton’s laws show active and passive masses are equal, and empirical measurements confirm inertial and gravitational mass match to one part in a trillion.
- field
- Physics
- unit
- Kilogram (kg)
- defined_by
- Planck constant, speed of light, and definition of the second
- key_principle
- Equivalence of inertial and gravitational mass
- known_for
- Resistance to acceleration and gravitational attraction
Lore & Background
Mass can be experimentally defined as a measure of a body's inertia, meaning the resistance to acceleration when a net force is applied. In the Standard Model of physics, the mass of elementary particles is believed to be a result of their coupling with the Higgs boson in what is known as the Brout–Englert–Higgs mechanism. There are several distinct phenomena used to measure mass: inertial mass, active gravitational mass, and passive gravitational 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.
Reader's Guide
Mass is an intrinsic positive physical quantity of a body that measures its resistance to acceleration. In modern physics, it is generally defined as the strength of an object's gravitational attraction to other bodies, as measured by an observer moving at the same speed. The SI unit of mass is the kilogram. Since 1905, mass has also been understood as a measure of a body's energy content, expressed through the relation E=mc². Mass can be experimentally defined as a measure of a body's inertia—its resistance to a change in velocity when a net force is applied. It is not the same as weight, which is a force; an object on the Moon weighs less than on Earth due to lower gravity, but its mass remains unchanged. In the Standard Model, the mass of elementary particles is believed to result from their coupling with the Higgs boson via the Brout–Englert–Higgs mechanism. Several distinct phenomena can measure mass, including inertial mass (resistance to acceleration), active gravitational mass (strength of the gravitational field generated), and passive gravitational mass (force exerted on an object in a known gravitational field). Experiments since the 17th century have shown inertial and gravitational mass to be identical, a principle incorporated into general relativity's equivalence principle. The kilogram was first defined in 1795 as the mass of one cubic decimetre of water at the melting point of ice, later redefined in 1889 as a metal prototype, and again in 2019 using only invariant constants: the speed of light, the caesium hyperfine frequency, the Planck constant, and the elementary charge. Non-SI units still widely used include the tonne, the dalton (unified atomic mass unit), the electronvolt (used in high-energy physics), the pound, the Planck mass, and the solar mass (used in astronomy). Newtonian gravity distinguishes between active and passive gravitational mass, but Newton's laws show they are the same, and empirical measurements confirm inertial and gravitational mass are identical to one part in a trillion.
Did You Know?
- Mass is not the same as weight; an object on the Moon would weigh less than on Earth but still have the same mass.
- The unit of mass in the International System of Units (SI) is the kilogram (kg).
- In the Standard Model, the mass of elementary particles is believed to result from their coupling with the Higgs boson.
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