Gravity and Newton's Laws                                                                                                                                                                   (The  above picture was taken from http://www.huntington.org/LibraryDiv/Newton/Newtonexhibit.htm)     Learning from the works of Galileo and Descartes, Newton was well aware of the principle of inertia, which says that once a body is set in to motion it will not stop until affected by some outside force. Newton used this to deduce what gravity is and how it works. He stated that an object fired into space will orbit the earth just like the moon. He called this work “A Treatise of the System of the World.”2 Newton later formed three laws of motion from his work with gravitation: 3  Every Body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. 3 The change of motion is proportional to the motive force impressed. It is made in the direction of the straight line in which the force is impressed. 3  To every action there is always opposed an equal reaction: or, the mutual actions of two bodies, each upon the other, are always equal and directly opposed.3