Galileos idea of a fixed value for the gravitational acceleration on the Earth iss now accepted for most pratical calulations allthough variations in the earths crust mean that this value is not precisely uniform. However there is another phenomenon that will affected the decent of the cannon balls. If the cannon balls were made of a very light material there would be a very noticble difference in the fall time for different sized balls. Why is this? Please involve calculations if possible. I would much appreciate any form of help.