| Weight | |
|---|---|
 A diagram explaining the mass and weight | |
Common symbols | |
| SI unit | newton (N) |
Other units | pound-force (lbf) |
| In SI base units | kg⋅m⋅s−2 |
| Extensive? | Yes |
| Intensive? | No |
| Conserved? | No |
Derivations from other quantities | |
| Dimension | |
In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition.[1][2][3]
Some standard textbooks[4] define weight as a vector quantity, the gravitational force acting on the object. Others[5][6] define weight as a scalar quantity, the magnitude of the gravitational force. Yet others[7] define it as the magnitude of the reaction force exerted on a body by mechanisms that counteract the effects of gravity: the weight is the quantity that is measured by, for example, a spring scale. Thus, in a state of free fall, the weight would be zero. In this sense of weight, terrestrial objects can be weightless: so if one ignores air resistance, one could say the legendary apple falling from the tree[citation needed], on its way to meet the ground near Isaac Newton, was weightless.
The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton.[1] For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, and about one-sixth as much on the Moon. Although weight and mass are scientifically distinct quantities, the terms are often confused with each other in everyday use (e.g. comparing and converting force weight in pounds to mass in kilograms and vice versa).[8]
Further complications in elucidating the various concepts of weight have to do with the theory of relativity according to which gravity is modeled as a consequence of the curvature of spacetime. In the teaching community, a considerable debate has existed for over half a century on how to define weight for their students. The current situation is that a multiple set of concepts co-exist and find use in their various contexts.[2]
- ^ a b Richard C. Morrison (1999). "Weight and gravity - the need for consistent definitions". The Physics Teacher. 37 (1): 51. Bibcode:1999PhTea..37...51M. doi:10.1119/1.880152.
- ^ a b Igal Galili (2001). "Weight versus gravitational force: historical and educational perspectives". International Journal of Science Education. 23 (10): 1073. Bibcode:2001IJSEd..23.1073G. doi:10.1080/09500690110038585. S2CID 11110675.
- ^ Gat, Uri (1988). "The weight of mass and the mess of weight". In Richard Alan Strehlow (ed.). Standardization of Technical Terminology: Principles and Practice – second volume. ASTM International. pp. 45–48. ISBN 978-0-8031-1183-7.
- ^ Knight, Randall D. (2004). Physics for Scientists and Engineers: a Strategic Approach. San Francisco, US: Addison–Wesley. pp. 100–101. ISBN 0-8053-8960-1.
- ^ Bauer, Wolfgang; Westfall, Gary D. (2011). University Physics with Modern Physics. New York: McGraw Hill. p. 103. ISBN 978-0-07-336794-1.
- ^ Serway, Raymond A.; Jewett, John W. (2008). Physics for Scientists and Engineers with Modern Physics. US: Thompson. p. 106. ISBN 978-0-495-11245-7.
- ^ Hewitt, Paul G. (2001). Conceptual Physics. US: Addison–Wesley. pp. 159. ISBN 0-321-05202-1.
- ^ The National Standard of Canada, CAN/CSA-Z234.1-89 Canadian Metric Practice Guide, January 1989:
- 5.7.3 Considerable confusion exists in the use of the term "weight". In commercial and everyday use, the term "weight" nearly always means mass. In science and technology "weight" has primarily meant a force due to gravity. In scientific and technical work, the term "weight" should be replaced by the term "mass" or "force", depending on the application.
- 5.7.4 The use of the verb "to weigh" meaning "to determine the mass of", e.g., "I weighed this object and determined its mass to be 5 kg," is correct.