update newtonian gravity page

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diff --git a/examples/newtonian_gravity/examples.toml b/examples/newtonian_gravity/examples.toml
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+
+[examples]
+
+  [examples.earth_surface]
+  r = 6.371e6       # radius of earth
+  m1 = 5.972e24     # mass of earth
+
+  [examples.earth_moon]
+  r = 385000        # radius of earth
+  m1 = 5.972e24     # mass of earth
+  m2 = 7.348e22     # mass of moon
+
diff --git a/examples/newtonian_gravity/model.modelica b/examples/newtonian_gravity/model.modelica
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+model NewtonianGravitation
+  "Simple/classical Inverse-square law force"
+  parameter Real    G=6.674e-11             "Gravitational Constant";
+  Real              m_1(unit="kilogram")    "mass #1";
+  Real              m_2(unit="kilogram")    "mass #2";
+  Real              r(unit="meter")         "radius";
+  Real              F                       "force";
+equation
+  F = (G * m_1 * m_2) / (r^2);
+end NewtonianGravitation;
diff --git a/examples/newtonian_gravity/page.md b/examples/newtonian_gravity/page.md
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+
+Newton's law of universal gravitation states that a particle attracts every
+other particle in the universe using a force that is directly proportional to
+the product of their masses and inversely proportional to the square of the
+distance between them. This is a general physical law derived from empirical
+observations by what Isaac Newton called inductive reasoning. It is a part of
+classical mechanics and was formulated in Newton's work Philosophiæ Naturalis
+Principia Mathematica ("the Principia"), first published on 5 July 1687. (When
+Newton's book was presented in 1686 to the Royal Society, Robert Hooke made a
+claim that Newton had obtained the inverse square law from him; see the History
+section below.)
+
+In modern language, the law states: Every point mass attracts every single
+other point mass by a force pointing along the line intersecting both points.
+The force is proportional to the product of the two masses and inversely
+proportional to the square of the distance between them. The first test of
+Newton's theory of gravitation between masses in the laboratory was the
+Cavendish experiment conducted by the British scientist Henry Cavendish
+in 1798. It took place 111 years after the publication of Newton's Principia
+and approximately 71 years after his death.
+
+Newton's law of gravitation resembles Coulomb's law of electrical forces, which
+is used to calculate the magnitude of the electrical force arising between two
+charged bodies. Both are inverse-square laws, where force is inversely
+proportional to the square of the distance between the bodies. Coulomb's law
+has the product of two charges in place of the product of the masses, and the
+electrostatic constant in place of the gravitational constant.
+
+
+## Alternatives
+
+Newton's law has since been superseded by Albert Einstein's theory of general
+relativity, but it continues to be used as an excellent approximation of the
+effects of gravity in most applications. Relativity is required only when there
+is a need for extreme precision, or when dealing with very strong gravitational
+fields, such as those found near extremely massive and dense objects, or at
+very close distances (such as Mercury's orbit around the Sun).
+
+### Observational Foils
+
+Newton's Theory does not fully explain the precession of the perihelion of the
+orbits of the planets, especially of planet Mercury, which was detected long
+after the life of Newton. There is a 43 arcsecond per century discrepancy
+between the Newtonian calculation, which arises only from the gravitational
+attractions from the other planets, and the observed precession, made with
+advanced telescopes during the 19th century.
+
+The predicted angular deflection of light rays by gravity that is calculated by
+using Newton's Theory is only one-half of the deflection that is actually
+observed by astronomers. Calculations using General Relativity are in much
+closer agreement with the astronomical observations.
+
+In spiral galaxies, the orbiting of stars around their centers seems to
+strongly disobey Newton's law of universal gravitation. Astrophysicists,
+however, explain this spectacular phenomenon in the framework of Newton's laws,
+with the presence of large amounts of Dark matter.
+
+## Solutions to the equation
+
+The n-body problem is an ancient, classical problem of predicting the
+individual motions of a group of celestial objects interacting with each other
+gravitationally. Solving this problem — from the time of the Greeks and on —
+has been motivated by the desire to understand the motions of the Sun, planets
+and the visible stars. In the 20th century, understanding the dynamics of
+globular cluster star systems became an important n-body problem too. The
+n-body problem in general relativity is considerably more difficult to solve.
+
+The classical physical problem can be informally stated as: given the
+quasi-steady orbital properties (instantaneous position, velocity and time) of
+a group of celestial bodies, predict their interactive forces; and
+consequently, predict their true orbital motions for all future times.
+
+The two-body problem has been completely solved, as has the Restricted 3-Body
+Problem.
+
+## References
+
+The body of this page originally came from Wikipedia.
+
+* Wikipedia: [Gravity](https://en.wikipedia.org/wiki/Gravity),
+  [Newton's law of universal gravitation](https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation),
+  [Inverse Square Law](https://en.wikipedia.org/wiki/Inverse-square_law)
+