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 the term weight is older, and is still used in metrology and in common usage. The second meaning of the term weight is more recent, and is used mainly by scientists, and is also in common usage is well. Many scientists and people with a scientific education like to claim that only the second meaning is correct, however the historical evidence shows otherwise. (Since time immemorial weight has 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 only a recent development.) On the other hand the CGPM reccomends the use of the word 'weight' to refer only to force, and not to mass. (Declaration of the 3rd CGPM, 1901, CR 70).

There is a related confusion that pounds are a unit of weight and kilograms are a unit of mass. In the past, there were conflicting two conflicting definitions of the pound, one as a unit of force (in which case slugs were used for mass), and the other as a unit of mass (in which case pound-forces or poundals were used for force). However, today the pound is legally defined in the United States as a unit of mass. According to the present deifinition, then, both kilograms and pounds are both measures of mass, and hence also units of weight (sense 1). The Imperial or U.S. customary unit for force, i.e. weight sense 2, is pounds-force?, not pounds, although pounds-force is often abbreviated to pounds.

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