All masses attract one another with a gravitational force proportional to their masses and inversely proportional to the square of the distance between them.
Spherically symmetrical masses can be treated as if all their mass were located at the center.
Nonsymmetrical objects can be treated as if their mass were concentrated at their center of mass, provided their distance from other masses is large compared to their size.
The weight of an object is the gravitational attraction between Earth and the object.
The gravitational field is represented as lines that indicate the direction of the gravitational force; the line spacing indicates the strength of the field.
Apparent weight differs from actual weight due to the acceleration of the object.
Orbital velocities are determined by the mass of the body being orbited and the distance from the center of that body, and not by the mass of a much smaller orbiting object.
The period of the orbit is likewise independent of the orbiting object’s mass.
Bodies of comparable masses orbit about their common center of mass and their velocities and periods should be determined from Newton’s second law and law of gravitation.
All orbital motion follows the path of a conic section. Bound or closed orbits are either a circle or an ellipse; unbounded or open orbits are either a parabola or a hyperbola.
The areal velocity of any orbit is constant, a reflection of the conservation of angular momentum.
The square of the period of an elliptical orbit is proportional to the cube of the semi-major axis of that orbit.