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Gravity field surrounding Earth from a macroscopic perspective. Newton's law of universal gravitation can be written as a vector equation to account for the direction of the gravitational force as well as its magnitude. In this formula, quantities in bold represent vectors.
dA is a vector, whose magnitude is the area of an infinitesimal piece of the surface ∂V, and whose direction is the outward-pointing surface normal (see surface integral for more details), g is the gravitational field, G is the universal gravitational constant, and; M is the total mass enclosed within the surface ∂V.
universal gravitational constant: newton meter squared per kilogram squared (N⋅m 2 /kg 2) shear modulus: pascal (Pa) or newton per square meter (N/m 2) gluon field strength tensor: inverse length squared (1/m 2) acceleration due to gravity: meters per second squared (m/s 2), or equivalently, newtons per kilogram (N/kg)
The second term in the above equation, plays the role of a gravitational force. If f f α {\displaystyle f_{f}^{\alpha }} is the correct expression for force in a freely falling frame ξ α {\displaystyle \xi ^{\alpha }} , we can use then the equivalence principle to write the four-force in an arbitrary coordinate x μ {\displaystyle x^{\mu }} :
This is the formula for the relativistic doppler shift where the difference in velocity between the emitter and observer is not on the x-axis. There are two special cases of this equation. The first is the case where the velocity between the emitter and observer is along the x-axis. In that case θ = 0, and cos θ = 1, which gives:
The gravitational constant appears in the Einstein field equations of general relativity, [4] [5] + =, where G μν is the Einstein tensor (not the gravitational constant despite the use of G), Λ is the cosmological constant, g μν is the metric tensor, T μν is the stress–energy tensor, and κ is the Einstein gravitational constant, a ...
Combining these ideas gives a formula that relates the mass and the radius of the Earth to the gravitational acceleration: = ^, where the vector direction is given by ^, is the unit vector directed outward from the center of the Earth.
By definition, all Euclidean vectors have a magnitude (see above). However, a vector in an abstract vector space does not possess a magnitude. A vector space endowed with a norm, such as the Euclidean space, is called a normed vector space. [8] The norm of a vector v in a normed vector space can be considered to be the magnitude of v.