Weight
Weight is the force exerted upon an object by virtue of its position in a gravitational field. In a constant gravitational field, such as the Earth's, this force is proportional to the object's mass. Hence, quantitatively, it is given by product of the mass of the object by the acceleration due to gravity; W = m × g. The SI unit is the newton (N) or kilogram metres per second squared (kg · m · s−2).
The terms mass and weight are often used interchangeably and indeed went historically undistinguished. The CGPM recommends that the word 'weight' be used to refer only to force, and not to mass. The verb 'to weigh' may still be used for mass determinations, although 'to mass' or 'to measure the mass' is also used.
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 common in engineering, and it occurs in derived units like p.s.i. (pounds per square inch). In most countries, scientists have adopted SI units, which use kilogram for mass and newton for force non-interchangeably.
The weight force that we sense is actually the normal reaction force, which prevents us from being pulled to the centre of the Earth. In contrast, in free-fall, there is no apparent weight because we are not in contact with any surface to provide such a reaction force. The experience of having no weight is known as weightlessness or microgravity.
Comparative weights on bodies of the solar system
The following is a list of the weights of a mass on some of the bodies in the solar system, relative to its weight on Earth:
For weight variations on Earth, see gee, physical geodesy and gravity anomaly.
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