math

# A Matter of Gravity

Nothing beats out a great new idea like a well-established mediocre idea. This holds true in the history of science, and in particular, it holds true for our understanding of gravity.

In Aristotle's (384 BC - 322 BC) physics, the concept of gravity was tied up with his concept of the four elements. Each element had its place in the universe and gravity was the effect of objects seeking that proper place. There were unfortunately quite a number of additional incorrect ideas that were tied up in this explanation. Each of these ideas had to be individually challenged before an accurate theory of gravity could be developed.

## The Earth is the center of the universe.

Nicolaus Copernicus' (1473 - 1543) great contribution to science was to challenge Aristotle's notion that the Earth must be at a special place at the center of the universe. In Copernicus' heliocentric universe, the Earth and planets rotated around the Sun.

Galileo Galilee (1564 - 1642) later used his telescope to substantiate Copernicus' heliocentric model. He viewed the phases of Venus, demonstrating that Venus revolves around the Sun rather than around the Earth as Aristotle believed.

## Gravity is no more than an object seeking its place among the four elements.

René Descartes (1596 - 1650) was careful not to weigh in on Copernicus' heliocentric theory, but he did offer an alternative to Aristotle's explanation of gravity. Descartes viewed gravity as the result of the aether pushing down as it sped around the Earth, in much the same way that a water droplet is shaped by the air. Christian Huygens (1629 - 1695) took this a step further and calculated that the aether must rotate around the Earth at 17 times the speed of the Earth's rotation in order to impart a force equal to that of gravity.

We know today that the aether does not exist, so the aether theory of gravity is incorrect. But, these two Renaissance scientists took another chip out of Aristotle's physics.

## The speed of a falling object is proportional to its weight.

Galileo provided much to our understanding of the effect of gravity. He is said to have dropped balls from the Leaning Tower of Pisa to demonstrate that Aristotle was wrong about the speed of a falling body, and showed that the speed of descent of an object is proportional to the time of the fall and not to weight.

## Everything in the heavens is perfect and unchanging.

Aristotle drew a firm distinction between the physics of the Earth and the physics of the heavens. Galileo's telescope showed craters on the Moon, bringing into question the notion of celestial perfection.

## Celestial bodies move in circles.

Johannes Kepler (1571 - 1630) spent years trying to reconcile observational data about the orbit of Mars with the idea that orbits had to be based on circles. He finally gave up on circles and tried an ellipse. This proved to be a simple solution. Kepler's first law says that planets travel about the Sun in elliptical orbits.

## Gravity does not apply to the heavens.

It fell on Isaac Newton's (1643 - 1727) shoulders to challenge the idea that celestial objects were somehow fundamentally different from Earthly objects. It seems obvious that the stars must be immune to the effects of gravity. How often do we see a star fall to Earth? Newton was able to explain that the balance between the force of gravity and momentum is precisely what holds the stars in the sky.

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# What is the physical significance of acceleration due to gravity?

This affects, for example, how quickly something that you let fall  will reach the ground.   Also, the acceleration due to gravity (in meters per square second)  is equi (MORE)
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# What is gravity in physics?

9.8 m/s. It is the mutual attraction between two masses
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