Weight

For the 1994 album by the group Rollins Band, see Weight (album).

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 force exerted by the surface we stand on, which prevents us from being pulled to the centre of the Earth and not, the weight itself. This normal force, that we can call the apparent weight is the one that is measured by a weighing scale, not the weight itself. A good evidence of this is given by the fact that a person moving up and down on his toes does see the indicator moving, telling that the measured force is changing while his weight, that depends only on his mass, the Earth mass and the distance between his center of mass and the center of Earth obviously do no change.

In contrast, in free-fall, there is no apparent weight because we are not in contact with any surface to provide such a normal force. The experience of having no apparent 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.

Related articles

• Weights and measures
• Historical weights and measures
• Atomic weight
• Human weight

External links

• Interesting discussion about the differences between weight and mass (http://ourworld.compuserve.com/homepages/Gene_Nygaard/weight.htm)
• Conversions: English and American weight units to metric units (http://www.sengpielaudio.com/ConvWeig.htm)
• Weight and mass conversion, weight of water, liters, volume units and capacity measures (http://www.sengpielaudio.com/calculator-milligram.htm)

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