[I am new to this forum and I am sorry if posted this in the wrong topic.] I just wondered if you could help me with the following (I know it's long :-( ): I had the idea of calculating g (gravitational force) on Earth using a marble (spherical) and a slant board (a hard board that is tilted; theoretically a plane at some angle alpha to the horizontal). I would measure the time for the marble to go down the plane and the distance it goes down and then calculate the acceleration and get g from there. Now there is a problem, namely, friction. It is easy to calculate the acceleration if there is no friction, but this is impossible for a practical experiment. Now, a marble that has some friction rolls down the plane (rotating obviously), while a marble without friction glides it down without any rotation at all (it could be a cube and it would give the same result). I've done a simulation on my computer and it turned out - as I expected - that the marbles that had no friction were faster than the ones with friction. What is interesting, all the marbles I gave a friction coefficient from 0.02 to up to more than 2 rolled down with the same speed; the speed varied only when µ was between 0.000 and ~0.02. I found out that the marbles with such low coefficient of friction were not fully rolling, they were gliding and rolling. Assuming that the friction of the plane and the marble is big enough to make the marble roll fully (not glide at all), how can I take this rotation caused by the friction into account, when wanting to measure the acceleration to find g? Is it the rotation that slows down the marbles? Or is it only the friction? Why are the differences in speed of marbles with and without friction so big, when the contact area of a sphere is so small? How can I include this in my calculations. Thank you for reading it (it turned out to be even longer than I thought it would get). I hope you can help me.