This idea wasn’t clearly disproven until Galileo demonstrated that, aside from effects of air resistance, gravity makes all objects fall to the ground at the same rate (Figure 3.19).įigure 3.19 – Galileo realized that, contrary to ancient beliefs, all falling objects would fall at the same rate without air resistance. For example, it was commonly assumed that heavier objects must fall faster than lighter-weight objects. People of the ancient world never developed a clear understanding of exactly how gravity affects falling objects.They simply assumed that “the heavens” (which is what they called everything in the sky) operated by different rules than Earth.
As far as we know, the idea that gravity might also affect the Moon, planets, or stars never occurred to anyone in the ancient world. The ancient idea of gravity applied only to objects on Earth.Notice that this implies at least two major differences between the ancient idea of gravity and our modern ideas: Figure 3.18 – Gravity determines the directions of up and down on Earth, with down being toward the center of the Earth and up being away from it. In other words, gravity gave meaning to the notions of “up” and “down” (Figure 3.18). Because the ancient Greeks recognized that the world is round, they also recognized that gravity acted in a direction toward the center of the Earth. In ancient times, the concept of gravity meant nothing more than the tendency of objects to fall to the ground. Historically, it took a very long time for an adequate theory of gravity to be developed, and our theory of gravity may still be incomplete even today. For example, gravity really does hold us to Earth, it really does make objects fall when you drop them, and it really does keep Earth and other planets in orbit around the Sun.īut why does gravity do these things? This is where a theory of gravity comes in, by which we mean a model that can explain the many roles of gravity and predict how gravity will act in other situations. Gravity is a “fact” in the sense that there is no doubt that it is real and does many things.
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