T. Arny Student Resources Chapter 2 Summary A gravitational force exists between any two objects in the Universe. The strength of this force depends on the masses of the bodies and their separation.
Gravitational forces hold astronomical bodies together and in orbit about one another.
A body's inertia makes it remain at rest or move in a straight line at a constant speed unless the body is acted on by a net force. Thus, for a planet to orbit the Sun, the Sun's force of gravity must act on it. The law of inertia and Newton's other laws of motion, when combined with the law of gravity, allow us to relate the size and speed of orbital motion to the mass of the central body. The gravitational force exerted by a planet determines its surface gravity and escape velocity. The former determines your weight on a planet.