<P> In common 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 exerted on an object by gravity . In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass . The object's 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 . </P> <P> 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 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 . </P>

When are mass and weight the same and when are they different
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