Kilogram << KIHL uh gram >> is the base unit of mass in the International System of Units (SI), the modern metric system. The mass of an object is a measure of its inertia, its resistance to changes in its motion. People also refer to mass as the amount of matter in an object.
Since 1889, a cylinder of platinum and iridium—often referred to as Le Grand K—stored at the International Bureau of Weights and Measures (BIPM) in France served as the SI standard for a mass equal to exactly 1 kilogram. However, the mass of this cylinder changed slightly over time as it gained and lost molecules. In 2018, BIPM member nations approved a new standard for the kilogram based on Planck’s constant. In physics, Planck’s constant is a universal fundamental measure that cannot change over time. It is a value derived from the frequency of a wave of light in relation to the energy of a photon in that wave.
In commercial and everyday activities, the term weight is understood to mean mass, and people use the kilogram as a unit of weight. In this sense, a kilogram equals 2.205 pounds.
In science and technology, weight is the gravitational force on an object. The SI has a separate unit for force, the newton. At the surface of the earth, an object’s weight is proportional to its mass, as shown by the equation W = mg, where W is weight in newtons, m is mass in kilograms, and g is the acceleration due to gravity—9.8 meters per second per second.
