The equations of motion explained earlier in this chapter were first worked out by Galileo Galilei, who was born at Pisa in Italy in 1564. Galileo began his university career as a medical student, but later forsook medicine for the study of mathematics and physics. His outstanding ability earned him a lectureship by the time he was twenty-five years of age, and eventually he became professor at the University of Papua. Later in life he was imprisoned for teaching that the sun was at the center of the solar system. This, being contrary to the teaching of the Church at that time, was regarded as an attempt to undermine its authority.
In the study of mechanics, Galilee is particularly remembered for the work he did on the acceleration of falling bodies. The Greek philosopher Aristotle taught that the speed with which a body fell to the ground depended on its mass. This wrongful assertion had been accepted for centuries, but legend has it that Galileo tested the truth of the matter by a very simple experiment. He ascended the leaning tower of Pisa and, from the top, he simultaneously released three iron balls of different masses. They all reached the ground at the same time. So much for the legend. The truth is that the Flemish engineer and mathematician, Simon Stein published a book on mechanics in 1586 in which he describes an experiment with two balls of lead, one ten times the mass of the other. These, when dropped simultaneously from a height of ten metres, reached the ground at the same time. This was nearly twenty years before Galileo’s supposed experiment at Pisa. See Fig. 3.6. Z .
It has been found that the acceleration due to gravity for a freely falling body has an average value’ of about 9.8 m/s2. The- accepted symbol for the acceleration due to gravity is an italic letter “g” to distinguish it from the symbol for gram which is a Roman letter “g”.
It is very easy to draw a wrong conclusion from a single casual observation. A feather, for example, falls to the ground much more slowly than a stone. The fact is that air resistance as well as force of gravity govern the rate at which a body falls towards the earth. In the case of a light feather of large surface area the air resistance is very large compared with the force of gravity on it. If air resistance is eliminated the feather falls with the same acceleration as the stone. 1his was first demonstrated by Robert Boyle shortly after Galileo’s death. Using his newly invented air pump, Boyle removed the air from a tall glass jar containing a lead bullet and a feather. When the jar was inverted both bullet and feather reached the bottom of the jar simultaneously.