What is It Many phenomena can be explained by the use of actual observation and simple common sense. Many attempts have been done to provide a set of ideas about motion as introduced in Activity 1. One of the famous men who developed an explanation for motion, which lasted for almost 2000 years, was philosopher Aristotle (384-322. B.C.). His ideas be exerted on it; but when this force is removed, the object comes to rest. Another important man is Galileo Galilei. In his experiment, he let a perfectly round ball rolled down an inclined surface and prepared three set ups under ideal conditions (lack of friction and air resistance). The following were his the Greek were based on very common experiences. Anistotle formulated the idea that, for an object to move, a force must observations: 1. The ball is released at a certain height. It speeds up and, if it rolls up another inclined surface of the same slope, it slows down and reaches the same height (figure 2A). Figure 2A 2. In another set up (figure 2B), the ball is released from the same height as figure 2A. It speeds up and if it is rolling up another inclined surface that is not as steep as figure 2A, it slows down and reaches the same height. However, it covers a farther distance as it rolls up the slope. Figure 2B 3. In the last set up, the ball is released from the same height as figure 2A. This time, it continues to roll in a flat surface. The ball does not speed up nor slow down (see Figure 2C) as if it continues to move forever. Figure 2 C Galileo concluded that in the absence of friction and air resistance, the ball would continue rolling up to a height above the base equal to the height from where it was released On a flat surface, the ball would continue to move forever, since there would be no force to cause its state of motion to change. Galileo was the first to suggest that uniform speed straight-line motion was just as natural as at-rest state of motion. He called this tendency of an object to maintain its initial state of motion as inertia.​​