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Motion on Earth

Updated on March 25, 2012

The study of motion on earth is made more difficult because of the many forces which act on a moving object.

Motion through the air is not the same as motion in a vacuum, where there is no air. Stroboscopic photographs of a feather falling in air show a small acceleration of the feather at the start of the motion, followed by an almost constant speed. The same feather falling in a vacuum drops 'like a stone', since it now has only the gravitational pull of the earth acting on it.

The effect of the air is obvious in the case of the feather, but though less obvious with a heavier falling object, it is still there. It may be some consolation to you to know that if you fall out of an aeroplane, your speed does not continue to increase. You will, in fact, reach a point where your speed remains constant, called terminal velocity.

The word velocity is often used in the same way as speed, but it will be seen later that velocity does not quite mean the same thing. Sky divers are able to use the effect of the atmosphere in this way and can, on reaching their terminal velocity, link hands and all fall at this same constant velocity. They would all be traveling at well over 147 feet per second.

We can, with the help of modern equipment, examine the motion of bodies under the influence of various forces, and then, to make things easier to understand, set up experiments to see how each force, on its own, affects the motion. This scientific method of discovery was not so easy in earlier times and it is not surprising that scientists often came to the wrong conclusions. This was because they were forced to examine very complicated motions, involving a number of effects, all at the same time.

From Aristotle to Galilee The Greek philosopher Aristotle considered rest to be the natural state of things on earth and that everything was made of some mixture of what he called 'the elements'- earth, water, air and fire. Each of these elements had a 'natural place' and when disturbed would always return to this natural resting place. A stone, being part of earth, would return to earth, but air and fire had resting places above the earth and would always rise above it.

These ideas led Aristotle to believe that heavier objects would fall faster than light ones. He would certainly have been surprised had he seen a modern stroboscopic photograph of a stone and a feather falling in a vacuum with the same acceleration. Aristotle's teachings, although wrong, were hardly doubted for many hundreds of years and no one took the trouble to perform experiments to test these theories, since it was thought that true understanding could only be obtained by reasoning things out. Asking why things happened, without first observing how they happened, was the method most used until the Middle Ages.

It was Galileo, considered by many to be the first modern scientist, who questioned this method of discovery. He concerned himself with observations and experiments which eventually led others to an understanding of why things move as they do. Galileo made the first scientific observations on freely falling bodies, distinguished between constant speeds and accelerations, and stated that 'within a finger or two' bodies of different weight released at the same time would fall equal distances in equal times.

There is some doubt whether he personally ever performed any experiments by dropping objects from the top of the leaning tower of Pisa, but he certainly encouraged others to challenge Aristotle's belief that heavy objects fall faster than light ones. One instance of this was in 1586, when a Dutch scientist, Simon Stevinus (1548-1620), reported an experiment with two lead balls, one ten times the weight of the other. He found that when released from the same height, the balls fell upon a hollow board and 'gave the same sound', indicating that they had fallen at the same speed. Galileo, hampered by lack of accurate timing equipment, found he could not investigate the motion of freely falling bodies as he would have liked, so he ingeniously slowed down the motion by experimenting on bodies rolling down slopes. He would have been pleased to see a modern photograph of this type of motion which confirms one of his conclusions: the distances traveled depend on the squares of the times taken. This special kind of motion, under the influence of the earth's gravitation, is called a constant acceleration. This does assume, however, that the air has little effect, or that the body is moving in a vacuum.


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