Weight

In physics, the weight of an object refers to the force exerted upon it due to gravity. In a constant gravitational field, like the Earth's, this force is proportional to the object's mass, and as a result the terms are often used interchangeably and indeed went historically undistinguished. It was only after the development of modern science that a distinction between the terms became important. The CPGM recommends that the word 'weight' be used to refer only to force, and not to mass.

Related to the historical identification of mass and weight, the pound has been used both as a unit of mass and as a unit of force. In the United States, United Kingdom, and elsewhere, the pound is now officially defined as a unit of mass. The corresponding force is called a pound-force, and similarly the weight of a kilogram of material on Earth is called a kilogram-force. However the use of pounds to measure forces is still very common in physics and engineering, and in derivative units like p.s.i. (pounds per square inch).

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