Why were experimental variables (like tilt) so important?
Galileo described his experiment with an inclined plane in Two New Sciences. In this work, Galileo furthered a research tradition in physics known as "impetus." This tradition, begun in the 6th century CE with John Philoponos, sought to explain motion. A body falling straight down accelerates too fast to be measured, so Galileo constructed his inclined plane in order to study it in slow motion. Galileo's inclined plane provided experimental confirmation of the law of free fall.