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( Nanogram)
The kilogram or kilogramme[Note 1] (symbol kg) is the base unit of mass in the International System of Units (SI, from the French Le Système International d’Unités). The kilogram is defined as being equal to the mass of the International Prototype Kilogram[1] (IPK),[Note 2] which is almost exactly equal to the mass of one liter of water. It is the only SI base unit with an SI prefix as part of its name. It is also the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property that can be reproduced in different laboratories. In everyday usage, the mass of an object, which is measured in kilograms, is often referred to as its weight, although in scientific context the term weight properly refers the gravitational force on an object, the SI unit for which is the newton (but the non-SI unit kilogram-force is also used). Similarly, the avoirdupois pound, used in both the Imperial system and U.S. customary units, is a unit of mass and its related unit of force is the pound-force. The avoirdupois pound is defined as exactly 0.45359237&_160;kg,[2] making one kilogram approximately equal to 2.2046 avoirdupois pounds. Many units in the SI system are defined relative to the kilogram so its stability is important. After the International Prototype Kilogram had been found to vary in mass over time, the International Committee for Weights and Measures (known also by its French-language initials CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature.[3] No final decision is expected before 2011.[4] The kilogram is a unit of mass, the measurement of which corresponds to the general, everyday notion of how “heavy” something is. However, mass is actually an inertial property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force. An object with a mass of one kilogram will accelerate at one meter per second squared (about one-tenth the acceleration due to Earth’s gravity) when acted upon by a force of one newton (symbol&_160;N).
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