Galileo Galilei
Buckets, jars and balls
During his time at Pisa, Galileo allegedly dropped two cannon balls – one heavier than the other – from the top of the Leaning Tower. Aristotle had said that objects fall with a speed proportional to their weight. Thus the heavier ball should travel faster than the lighter one. Galileo hypothesised that the balls would both accelerate as they fell but would hit the ground at the same instant.
But this experiment, if it was ever carried out, was doomed to failure. Air resistance meant that the lighter ball would always fall more slowly. And everything would have happened so fast that it would have been impossible to do accurate measurements.
A fundamental law
It was another crucial experiment carried out in about 1604 that proved Galileo's point. This involved some very simple apparatus but led to a fundamental law of engineering and mechanics.
Galileo thought that, if he could slow the motion of the balls, he would not only reduce the effect of air resistance but would also be able to carry out accurate measurements. His solution? He used a slope, or inclined plane.
He made a wooden ramp about 7 metres (23 feet) long and 30 centimetres (12 inches) wide. Along its entire length was a straight groove a little wider than a finger's width, which was so well polished that a bronze ball could roll down it easily with little friction.
Hole in a bucket
Galileo also had to find a way to measure time accurately – not an easy task in an era when there were no accurate clocks. (In fact, it was Galileo's own work on the pendulum that would lead to the development of modern clocks.)
He made a hole in the bottom of a bucket and suspended a jar immediately below it. Blocking the hole with a finger, he filled the bucket with water. The moment he let a ball go down the ramp, he unblocked the hole and water flowed into the jar. When the ball hit a block at a set point on the ramp, he again blocked the hole and then weighed the water in the jar. In effect, the bucket and jar comprised a rudimentary stopwatch, the weight of the water in the jar representing the time it took the ball to roll down the ramp.
Rolling, rolling, rolling
Again and again, Galileo rolled the ball down the slope, measured (in water) the time it took to reach marked divisions on the slope and then compared the times. When he had collated enough data, he did a few simple calculations until he was able to work out a rule about the ball's motion: a falling body travels a distance that is a square of the time taken.
In other words, if it took 1 second for a ball to travel 1 unit of length, it would take it 4 seconds to travel 16 units. This meant that the ball did not travel at a constant rate but accelerated. Galileo also found that the weight of the ball did not matter – balls of different weights all arrived at the end of the ramp in the same time.
Words versus eyes
Although he had proved Aristotle wrong, Galileo was instructed by his academic masters to ignore his own findings. He responded sarcastically, 'If experiments are performed thousands of times in all seasons and in every place without once producing the effect described by the philosophers, poets and historians, this means nothing and we must believe their words and not our own eyes.'
