ABOUT SCIENCE

Galileo’s Concept of Inertia | Describing Motion - Aristotle’s ideas were accepted as fact for nearly 2,000 years. Then, in the early 1500s, the Italian scientist Galileo demolished Aristotle’s belief that heavy objects fall faster than light ones. According to legend, Galileo dropped both heavy and light objects from the Leaning Tower of Pisa (Figure 1). He showed that, except for the effects of air friction, objects of different weights fell to the ground in the same amount of time.

Galileo made another discovery. He showed that Aristotle was wrong about forces being necessary to keep objects in motion. Although a force is needed to start an object moving, Galileo showed that, once it is moving, no force is needed to keep it moving except for the force needed to overcome friction. When friction is absent, a moving object needs no force to keep it moving. It will remain in motion all by it self. Rather than philosophizing about ideas, Galileo did something that was quite remarkable at the time. 

Galileo tested his revolutionary idea by  experiment. This was the beginning of modern science. He rolled balls down inclined planes and observed and recorded the gain in speed as rolling continued (Figure 2). On downward-sloping planes, the force of gravity increases a ball’s speed. On an upward slope, the force of gravity decreases a ball’s speed. What about a ball rolling on a level surface? While rolling on a level surface, the ball neither rolls with nor against the vertical force of gravity it neither speeds up nor slows down. The rolling ball maintains a constant speed. Galileo reasoned that a ball moving horizontally would move forever, if friction were entirely absent (Figure 3). Such a ball would move all by itself of its own inertia.