I am planning an experiment for my coursework in which i am dropping spheres of different mass and calculating their drag. They will be of different sizes, but are made of plastercine so are relatively light. I would guess the heaviest would weigh no more than 20 grams. Does anybody know roughly how far the heaviest of these balls would have to be dropped before it reaches terminal velocity? Because if possible i want to do my experiment in the classroom without having to drop something out the window and take my equipment outside. Any estimates would be much appreciated.