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If the Earth had a constant density ρ, the mass would be M(r) = (4/3)πρr 3 and the dependence of gravity on depth would be =. The gravity g′ at depth d is given by g′ = g(1 − d/R) where g is acceleration due to gravity on the surface of the Earth, d is depth and R is the radius of the Earth.
Cavendish's result was first improved upon by John Henry Poynting (1891), [24] who published a value of 5.49(3) g⋅cm −3, differing from the modern value by 0.2%, but compatible with the modern value within the cited relative standard uncertainty of 0.55%.
The value of ɡ 0 defined above is a nominal midrange value on Earth, originally based on the acceleration of a body in free fall at sea level at a geodetic latitude of 45°. Although the actual acceleration of free fall on Earth varies according to location, the above standard figure is always used for metrological purposes.
At a fixed point on the surface, the magnitude of Earth's gravity results from combined effect of gravitation and the centrifugal force from Earth's rotation. [2] [3] At different points on Earth's surface, the free fall acceleration ranges from 9.764 to 9.834 m/s 2 (32.03 to 32.26 ft/s 2), [4] depending on altitude, latitude, and longitude.
After converting to SI units, Cavendish's value for the Earth's density, 5.448 g cm −3, gives G = 6.74 × 10 −11 m 3 kg –1 s −2, [24] which differs by only 1% from the 2014 CODATA value of 6.67408 × 10 −11 m 3 kg −1 s −2. [25] Today, physicists often use units where the gravitational constant takes a different form.
For two bodies, the parameter may be expressed as G(m 1 + m 2), or as GM when one body is much larger than the other: = (+). For several objects in the Solar System, the value of μ is known to greater accuracy than either G or M. The SI unit of the standard gravitational parameter is m 3 ⋅s −2.
Gravity of Earth, the acceleration caused by the combination of gravitational attraction and centrifugal force of the Earth; Standard gravity, or g, the standard value of gravitational acceleration at sea level on Earth
Other units include the cgs gal (sometimes known as a galileo, in either case with symbol Gal), which equals 1 centimetre per second squared, and the g (g n), equal to 9.80665 m/s 2. The value of the g n is defined as approximately equal to the acceleration due to gravity at the Earth's surface, although the actual acceleration varies slightly ...