In the late seventeenth century, Sir Isaac Newton was able to complete what the Greeks had started: he unified the motion of the heavenly bodies with that of Earthly objects (such as the proverbial apple that fell on his head) within a single elegant framework, often called ``Newtonian mechanics". Newton first realized that objects change the state of their state of motion if and only if they are subjected to external influences: that is, they tend to continue moving in a straight line at uniform speed unless something forces them to do otherwise. The ball at the end of this string would go in a straight line if I weren't pulling it inward with this string. You can see that this is true if I release the string. So Newton deduced that when an apple accelerates towards the Earth (drop bean bag) it does so because the earth is pulling on it. Moreover, with an amazing example of intuition and logic, he figured out that it was the same external influence (the Earth's gravitational field) that caused the apple to fall, and kept the moon in orbit around the Earth.

As this diagram indicates, without the Earth's influence the moon would go in a straight line and escape from orbit: the difference between what the moon wants to do, and what the gravitational pull of the Earth forces it to do is an acceleration towards the center of the Earth that is identical to the acceleration of the apple. This was truly a remarkable feat of synthesis, and the beautiful structure created by Newton (Newtonian mechanics) stood unscathed for three hundred years.