Although never fully substantiated, a legend exists that explains how Sir Isaac Newton discovered gravity after watching an apple fall from a tree. There is some evidence though that this incident did in fact occur. William Stukeley, who was a colleague of Newton, and his assistant John Conduitt both wrote about the event taking place in manuscripts.
This event is often cited as the catalyst for Newton's discovery of gravitational force, which led to later discoveries, such as Newton's laws of motion. Newton’s law of gravity explains how all particles in the universe attract other particles with a force that is equal to the product of the particles' mass and the distance between the particles. This law is expressed in a mathematical equation used to calculate gravitational force.
The discovery of the causes and effects of gravitational force allowed Newton to create the three laws of motion. The first law of motion is that force is constant until it is interrupted by an external force. The second law is a mathematical equation used to calculate mass, acceleration and force. The last law of motion states: "For every action, there is an equal and opposite reaction." Newton published these discoveries and many others in 1687 in a book titled "Principia."