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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


The ball travels down one quarter of the plane in half the time it takes to
traverse the entire plane.


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.