Weight

The weight of an object refers in physics to the force exerted upon it due to gravity. This force is proportional to the object's mass.

The terms mass and weight are often used interchangeably. It was only after the development of modern science that a distinction between the terms became important.

History of the term

The first meaning of weight is older, and is still used in metrology. The second meaning of the term weight is more recent and is used mainly by scientists. Both are in common usage. Many people with scientific educations (including scientists) claim that the second meaning is correct, but historical evidence shows otherwise. (Weight has always been measured with balance scales, which measure mass, not force exerted due to gravity, although they require gravitiational force to operate -- spring balances, which measure force due to gravity, are a recent development.) The CGPM? recommends the use of the word 'weight' to refer only to force, and not to mass. (Declaration of the 3rd CGPM, 1901, CR 70).

A confusion arises that pounds are a unit of weight and kilograms are a unit of mass. In the past, there were two conflicting definitions of the pound, one as a unit of force (in which case slug?s were used for mass), and the other as a unit of mass (in which case poundal?s were used for force). In the United States, the pound is now officially defined as a unit of mass. So, according to the present definition, both kilograms and pounds are both measures of mass, and also units of weight (sense 1). The Imperial or U.S. customary unit for force, (sense 2), is pounds-force?, not pounds, although pounds-force is usually abbreviated to pounds.

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