Thestandard acceleration of gravity orstandard acceleration of free fall, often called simplystandard gravity, is the nominalgravitational acceleration of an object in avacuum near the surface of theEarth. It is a constant defined byISO standard 80000 as9.80665 m/s2 (about32.17405 ft/s2), denoted typically byɡ0 (sometimes alsoɡn,ɡe,[a] or simplyɡ). This value was established by the thirdGeneral Conference on Weights and Measures (1901, CR 70) and used to define the standardweight of an object as the product of its mass and this nominalacceleration.[2][3] The acceleration of a body near the surface of the Earth is due to the combined effects ofgravity andcentrifugal acceleration from the rotation of the Earth (but the latter is small enough to be negligible for most purposes); the total (the apparent gravity) is about 0.5% greater at thepoles than at theEquator.[4][5]
Although the symbolɡ is sometimes used for standard gravity,ɡ (without a suffix) can also mean the local acceleration due to local gravity and centrifugal acceleration, which varies depending on one's position on Earth (seeEarth's gravity). The symbolɡ should not be confused withG, thegravitational constant, or g, the symbol forgram. Theɡ is also used as a unit for any form of acceleration, with the value defined as above (see also:g-force).
The value ofɡ0 defined above is a nominal midrange value on Earth, originally based on the acceleration of a body in free fall at sea level at ageodetic latitude of 45°. Although the actual acceleration of free fall on Earth varies according to location, the above standard figure is always used formetrological purposes. In particular, since it is the ratio of thekilogram-force and thekilogram, its numeric value when expressed incoherent SI units is the ratio of the kilogram-force and thenewton, twounits of force.
Already in the early days of its existence, theInternational Committee for Weights and Measures (CIPM) proceeded to define a standardthermometric scale, using theboiling point of water. Since the boiling point varies with theatmospheric pressure, the CIPM needed to define a standard atmospheric pressure. The definition they chose was based on the weight of a column ofmercury of 760 mm. But since that weight depends on the local gravity, they now also needed a standard gravity. The 1887 CIPM meeting decided as follows:
The value of thisstandard acceleration due to gravity is equal to the acceleration due to gravity at the International Bureau (alongside thePavillon de Breteuil) divided by 1.0003322, the theoretical coefficient required to convert to a latitude of 45° at sea level.[6]
All that was needed to obtain a numerical value for standard gravity was now to measure the gravitational strength at theInternational Bureau. This task was given to Gilbert Étienne Defforges of the Geographic Service of the French Army. The value he found, based on measurements taken in March and April 1888, was 9.80991(5) m⋅s−2.[7]
This result formed the basis for determining the value still used today for standard gravity. The thirdGeneral Conference on Weights and Measures, held in 1901, adopted a resolution declaring as follows:
The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is 980.665 cm/s2, value already stated in the laws of some countries.[8]
The numeric value adopted forɡ0 was, in accordance with the 1887 CIPM declaration, obtained by dividing Defforges's result – 980.991 cm⋅s−2 in thecgs system thenen vogue – by 1.0003322 while not taking more digits than are warranted considering the uncertainty in the result.
| Base value | (Gal, or cm/s2) | (ft/s2) | (m/s2) | (standard gravity,g0) |
|---|---|---|---|---|
| 1 Gal, or cm/s2 | 1 | 0.0328084 | 0.01 | 1.01972×10−3 |
| 1 ft/s2 | 30.4800 | 1 | 0.304800 | 0.0310810 |
| 1 m/s2 | 100 | 1/0.3048 ≈3.28084 | 1 | 0.101972 |
| 1g0 | 980.665 | 32.1740 | 9.80665 | 1 |
γe = 9.780 326 7715 m/s² normal gravity at equator