Galileo's Inclined Plane Experiment

Using equipment that we might now call simple or even rudimentary, Galileo revolutionized basic scientific principles which were posited by Aristotle and held firmly by scholars of the High Middle Ages and Renaissance. One of his most important experiments was the inclined plane experiment. Galileo used his inclined plane, a simple board with a groove down which he rolled a small metal ball, to examine Aristotelian ideas about motion. Galileo's inclined plane experiment radically changed these ideas by concentrating on acceleration, a stage of motion ignored by Aristotle and most of his followers.

We decided to replicate Galileo's inclined plane experiment because it was so fundamental to new concepts of motion in Galileo's time. We based our experiment on Galileo's own description of the inclined plane in his book Discourses on Two New Sciences (1638):

A piece of wooden moulding or scantling, about 12 cubits [about 7 m] long, half a cubit [about 30 cm] wide and three finger-breadths [about 5 cm] thick, was taken; on its edge was cut a channel a little more than one finger in breadth; having made this groove very straight, smooth, and polished, and having lined it with parchment, also as smooth and polished as possible, we rolled along it a hard, smooth, and very round bronze ball.

Our own construction entailed planing at a 45 degree angle one edge each on two 16-foot two by fours, which when nailed together formed a groove. We sanded and oiled the groove to create a low-friction effect like Galileo's parchment. Instead of a small bronze ball, we used a three-quarter inch steel ball bearing. We added a metal piece to the end of the inclined plane, against which the ball struck at the end of each run, to make our timing precise. Click here for a more detailed description of our inclined plane.

We also replicated Galileo's apparatus for timing the inclined plane experiment. Galileo describes his water clock in Discourses on Two New Sciences (1638):

For the measurement of time, we employed a large vessel of water placed in an elevated position; to the bottom of this vessel was soldered a pipe of small diameter giving a thin jet of water, which we collected in a small glass during the time of each descent... the water thus collected was weighed, after each descent, on a very accurate balance; the difference and ratios of these weights gave us the differences and ratios of the times...

Our water clock consisted of a plastic bucket with a small hole drilled in the bottom, into which we placed a length of plastic tubing. When filled with water the bucket emitted a thin stream of water through the plastic tubing. We controlled the flow of water by clamping the tubing with a small metal clamp.

We marked our inclined plane at one quarter, one half, and three quarters its length. Starting with the full length of the plane, we rolled the ball twenty times down each length, timing each trial with our water clock. Like Galileo, we weighed the water from each trial so as to determine the ratio of times for each length.

Our experiment proved that Galileo could have attained the accuracy which he claimed for this experiment. Our findings also point clearly to the concept of acceleration: the ball travels down one quarter of the plane in half the time it takes to traverse the entire plane. Aristotle would have posited, of course, that the ball's time would be directly proportional to the distance it traveled. Andrew Irving has created graphs which clarify Galileo's discovery that the ratio of the distances traversed by the ball is proportional to the ratio of the squares of the time.

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Last revised April 12, 1995
Cary Clifford