Mass and weight

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Mass and weight are two similar concepts with an important distinction. The mass of an object indicates its inertia or resistance to acceleration. This quantity remains constant, even for a man in a spaceship or on the moon. Weight, however, depends on how close the object is to a planet. Objects on the moon weigh only 1/6 their earth weight.

This distinction is important in high school physics classes, but in ordinary life the terms for mass and weight are used interchangeably. We say, for example, that 10 pounds of rice weighs 4.5 kilograms. Strictly speaking, metric units such as kilograms and grams measure mass rather than weight. Thus, a bodybuilder's 100 kg barbell weighs 220 pounds on earth, but would weigh only 36 pounds on the moon.


For a more detailed treatment, see mass.

Mass is a measurement of the amount of matter in an object, and is recorded in units such as grams (metric) and pounds (imperial). The mass of a particular object is universal, that is it does not rely on the position of the object in space.


For a more detailed treatment, see weight.

Weight is a measurement of the force acting on an object due to the position of its mass in a gravitational field, it is measured in newtons and depends on an objects location in space (for example, a ball resting on the surface of the earth will have a higher weight than an identical ball resting on the surface of the moon, as the earth has a stronger gravitational field than the moon).