The study of motion by Galileo Galilei (1564-1642) in the late sixteenth and early seventeenth centuries and by Sir Isaac Newton in the mid-seventeenth century was one of the major cornerstones of modern Western experimental science. Over a period of 20 years, Galileo observed the motions of objects rolling down various inclines and attempted to time these events. He discovered that the distance an object traveled was proportional to the square of the time that it was in motion. From these experiments came the first correct concept of accelerated motion. Newton wanted to know why acceleration occurred. In order to produce a model that would help explain how the known universe of the seventeenth century worked, Newton had to give to science and physics the concept of a force which was mostly unknown at that time. With his second law of motion, he clearly demonstrated that acceleration is caused by an unbalanced force (commonly called a push or a pull) acting on an object. What we call gravity, Newton showed was nothing more than a special type of acceleration. The interaction of the acceleration of gravity on the mass of our body produces the force which is called weight. A general definition of mass is that it refers to the quantity of matter in a body.