Kilogram explained to kids

The kilogram is the unit of mass in the metric system, and is the base unit of mass in the International System of Units (SI). The kilogram was originally defined as the mass of one litre (cubic decimetre) of water. The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice. The kilogram is the only SI unit with an SI prefix (kilo-) as part of its name. The word kilogramme or kilogram is derived from the French kilogramme, which itself was a learned coinage, prefixing the Greek stem of χίλιοι (khilioi, "a thousand") to gramma (γράμμα, "a small weight"). The SI unit of area is the square metre (m2). The SI unit of volume is the cubic metre (m3). The kilogram is a unit of mass, a property which corresponds to the common perception of how "heavy" an object is. Mass is an inertial property; that is, an object will remain at rest or in uniform motion in a straight line unless acted upon by a force. The kilogram was originally defined as the mass of one litre (cubic decimetre) of water. The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice. In 1879, the International Bureau of Weights and Measures (BIPM) was founded, and one of its first tasks was to standardise the units of measurement. The kilogram was a natural choice for the base unit of mass, as it was already in common use. One litre of water has a mass of almost exactly one kilogram. It is very close to the actual value, but it is not exact. The reason for the slight discrepancy is that the density of water varies slightly with temperature. The original definition of the kilogram was based on the litre, but the value of the litre was not defined. The first proposal was to define the litre as the volume of one kilogram of water at 4 degrees Celsius. This was not acceptable, as it would make the temperature of water dependent on the kilogram. The current definition of the kilogram, approved in 1901, is based on the Planck constant. The Planck constant is a very small number, and so the kilogram is now defined as the mass of a very small number of particles called Planck particles. The Planck particle is a hypothetical particle that is believed to be the smallest unit of matter. It has never been observed, but its existence is predicted by the theory of quantum mechanics. The Planck constant is the fundamental constant of quantum mechanics. It is the smallest unit of action, and sets the scale for all other units of action in the universe. The value of the Planck constant is very small, and so the mass of a single Planck particle is also very small. This means that the kilogram is now defined as the mass of a very small number of particles called Planck particles.