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In everyday usage, the mass of an object is often referred to as its weight, though these are in fact different concepts and quantities. In scientific contexts, mass refers loosely to the amount of "matter" in an object (though "matter" may be difficult to define), whereas weight refers to the force experienced by an object due to gravity.〔de Silva, G.M.S. (2002), (Basic Metrology for ISO 9000 Certification ), Butterworth-Heinemann〕 In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons (newton is the unit of force, while kilogram is the unit of mass) on the surface of the Earth (its mass multiplied by the gravitational field strength). Its weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass. Objects on the surface of the Earth have weight, although sometimes this weight is difficult to measure. An example is a small object floating in a pool of water (or even on a dish of water), which does not appear to have weight since it is buoyed by the water; but it is found to have its usual weight when it is added to water in a container which is entirely supported by and weighed on a scale. Thus, the "weightless object" floating in water actually transfers its weight to the bottom of the container (where the pressure increases). Similarly, a balloon has mass but may appear to have no weight or even ''negative'' weight, due to buoyancy in air. However the weight of the balloon and the gas inside it has merely been transferred to a large area of the Earth's surface, making the weight difficult to measure. The weight of a flying airplane is similarly distributed to the ground, but does not disappear. If the airplane is in level flight, the same weight-force is distributed to the surface of the Earth as when the plane was on the runway, but spread over a larger area. A better scientific definition of mass is its description as being composed of inertia, which basically is the resistance of an object being accelerated when acted on by an external force. Gravitational "weight" is the force created when a mass is acted upon by a gravitational field and the object is not allowed to free-fall, but is supported or retarded by a mechanical force, such as the surface of a planet. Such a force constitutes weight.〔National Physical Laboratory: (What are the differences between mass, weight, force and load? (FAQ - Mass & Density) )〕 This force can be added to by any other kind of force. For example, in the photograph, the girl's weight, subtracted from the tension in the chain (respectively the support force of the seat), yields the necessary centripetal force to keep her swinging in an arc. If one stands behind her at the bottom of her arc and abruptly stops her, the impetus ("bump" or stopping-force) one experiences is due to acting against her inertia, and would be the same even if gravity were suddenly switched off. While the ''weight'' of an object varies in proportion to the strength of the gravitational field, its ''mass'' is constant (ignoring relativistic effects) as long as no energy or matter is added to the object.〔See Mass in special relativity for a discussion of mass in this context. An object or particle does not have to be traveling very close to the speed of light, ''c'', for its relativistic mass, ''M'' (or γ''m'') to measurably vary from its rest mass ''m''0. Per the Lorentz transformations and Einstein’s 1905 paper, ''The Special Theory of Relativity'', relativistic mass is 0.5% greater than ''m''0 at only 9.96% ''c'', thus affecting measurements performed at a precision of one percent. Whereas 10% the speed of light is exceedingly fast in most contexts, it is not "close to the speed of light".〕 Accordingly, for an astronaut on a spacewalk in orbit (a free-fall), no effort is required to hold a communications satellite in front of him; it is "weightless". However, since objects in orbit retain their mass and inertia, an astronaut must exert ten times as much force to accelerate a 10ton satellite at the same rate as one with a mass of only 1 ton. On Earth, a swing set can demonstrate this relationship between force, mass, and acceleration. If one were to stand behind a large adult sitting stationary on a swing and give him a strong push, the adult would temporarily accelerate to a quite low speed, and then swing only a short distance before beginning to swing in the opposite direction. Applying the same impetus to a small child would produce a much greater speed. ==Overview== Mass corresponds to the general, everyday notion of how "heavy" something is. Mass is (among other properties) an ''inertial'' property; that is, the tendency of an object to remain at constant velocity unless acted upon by an outside force. Under Sir Isaac Newton's -year-old laws of motion and an important formula that sprang from his work, an object with a mass, ''m'', of one kilogram will accelerate, ''a'', at one meter per second per second (about one-tenth the acceleration due to earth’s gravity)〔In professional metrology (the science of measurement), the acceleration of Earth’s gravity is taken as standard gravity (symbol: ''g''n), which is defined as precisely metersper square second (m/s2). The expression means that ''for every second that elapses'', velocity changes an additional 1 meter per second. An acceleration of 1m/s2 is the same rate of change in velocity as 3.6 km/h per second (≈2.2 mph per second).〕 when acted upon by a force, ''F'', of one newton. Inertia is seen when a bowling ball is pushed horizontally on a level, smooth surface, and continues in horizontal motion. This is quite distinct from its weight, which is the downwards gravitational force of the bowling ball one must counter when holding it off the floor. The weight of the bowling ball on the Moon would be one-sixth of that on the Earth although its mass remained unchanged. Consequently, whenever the physics of ''recoil kinetics'' (mass, velocity, inertia, inelastic and elastic collisions) dominate and the influence of gravity is a negligible factor, the behavior of objects remains consistent even where gravity is relatively weak. For instance, billiard balls on a billiard table would scatter and recoil with the same speeds and energies after a break shot on the Moon as on Earth; they would, however, drop into the pockets much more slowly. In the physical sciences, the terms "mass" and "weight" are rigidly defined as separate measures, as they are different physical properties. In everyday use, as all everyday objects have both mass and weight and one is almost exactly proportional to the other, "weight" often serves to describe both properties, its meaning being dependent upon context. For example, in retail commerce, the "net weight" of products actually refers to mass and is expressed in mass units such as grams or ounces (see also ''Pound: Use in commerce)''. Conversely, the load index rating on automobile tires, which specifies the maximum structural load for a tire in kilograms, refers to weight; that is, the force due to gravity. Before the late 20th century, the distinction between the two was not strictly applied in technical writing, so that expressions such as "molecular weight" (for molecular mass) are still seen. Because mass and weight are separate quantities, they have different units of measure. In the International System of Units (SI), the kilogram is the unit of mass, and the newton is the unit of force. The non-SI kilogram-force is also a unit of force typically used in the measure of weight. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mass versus weight」の詳細全文を読む スポンサード リンク
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