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Facts of the Matter

Richard Brill

Sunday, September 5, 2004


Notions of motion
owe debt to Galileo


I get a good laugh whenever I read a report that "speed was a factor" in a traffic accident.

Whenever there is motion there is speed. Therefore, speed is a factor in any accident where there is motion, and I would wager there are few traffic accidents that involve no motion.

The reporter probably means "excessive speed," but a scientist's point of view, misuse of the term is like hearing fingernails screech across a blackboard.

In the same vein, it could be said that even "excessive speed" is a relative term since there is nothing inherent in any speed that, taken by itself, will cause an accident. It is speed that is in excess of the degree of attentiveness or the ability to handle the vehicle that causes accidents, not speed per se. The oft-repeated phrase "speed kills" is equally misleading. It is not speed that kills, but rather the sudden stop, which admittedly is more severe the higher the speed and the more sudden the stop.

Pervasive as motion is in the world around us, its simplicity is deceptive, as evidenced by the fact that it was a mystery until a mere 400 years ago. Galileo studied it in the late 1500s and formulated the definitions in modern terms.

Although Galileo is more commonly known as the first to use the telescope for astronomical observations and for his disagreement with the church about the center of the universe, his work on motion is far more significant in the history of ideas.

To study motion, he conceived of the scientific method of hypothesis testing, and designed brilliant experiments to test his ideas. His studies of motion and his mathematical description of it paved the way for Newton's formulation of the laws of motion and gravitation upon which all of our modern science is based.

Galileo put things in perspective with simple and elegant definitions that today might seem intuitive but which had eluded a world dominated by the views of Aristotle that dated from Greece in the third century B.C.

Aristotle's problem in describing motion was his belief that change could not be described mathematically, and his difficulty in overcoming our natural childish intuitions about motion. By Galileo's time, mathematics had come a long way and he used them in an innovative and brilliant way to describe change creatively and elegantly, using ratios of distance and time to signify rate. Bringing time into the equation was the missing link and is one of those strokes of genius that we now look back upon and say, "Of course!"

As brilliant as Aristotle was, and as tight as was his system of the world, motion was one thing that he just didn't get. Many medieval scholars recognized that his views on motion didn't make much sense, but there was little scholarly interest in it, partly because it was too esoteric and partly because the medieval Zeitgeist dictated that such things were deigned by the Creator never to be known by mankind. His views, including the misunderstanding of motion, were incorporated into dogma of the church in the 13th century along with the rest of his philosophy into a tight and rigid philosophy.

As formulated by Galileo, speed is nothing more than the change in location with respect to time, expressed in common units such as miles per hour or meters per second.

The range of speeds we encounter in various regions of the universe is quite large, from the indiscernible sluggishness of geologic processes to the incomprehensibly large speed of light.

Some geologic processes might be imperceptibly slow on the human scale: Soil and rock creep downslope at a rate of one inch in 25 years, about the same rate at which a mountain range is uplifted. At that rate, Mount Everest would rise from sea level to its present height in about 10 million years, although in reality it takes much longer since a mountain range is being eroded at the same time it is being uplifted.

A snail's pace, although slow by human standards, is breakneck by comparison with the slower geological processes. The world's record holder in snail racing (yes, there really is such a race) burned up the turf at an astounding 0.005 mph. I don't know how fast the average snail moves, but it no doubt depends on the individual snail and its motivation.

In the midrange of speeds is the speed of sound, which in air at sea level is about 740 mph.

On the other end of the earthly speed spectrum are molecules, which are in constant thermal motion. At 80 degrees Fahrenheit a typical air molecule is moving at about 1,150 mph as it and trillions upon trillions of others collide with each other, with us and everything else billions of times every second to create atmospheric pressure.

Luckily, this is quite a bit less than the escape velocity of 25,000 mph, which is the speed necessary to escape Earth's gravity from Earth's surface.

At about 70 percent of the escape velocity is a low-orbit satellite such as the International Space Station. To stay in orbit, it must maintain a speed of 17,238 mph in its orbit 200 miles up.

A geostationary satellite that orbits Earth in 24 hours must travel at only about 3,050 mph at an altitude of 22,241 miles above Earth's surface, quite slow by space standards.

On the cosmic speed scale, our sun orbits the center of the Milky Way galaxy at about 490,000 mph. Although this is quite fast by earthly standards, it is only a minuscule 7 percent of the speed of light, which is, according to current understanding, the ultimate speed that anything can travel.

Galileo's definition of motion set the stage for our modern understanding of the physical laws that govern motion in the universe and has given us a basis to form such scales of comparison that boggle the mind and remind us of our place in the cosmological schema.




Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu

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