zloq wrote:My point was that you said "exactly equal" twice in a single note - and all along I have been saying the difference in fall time is nonzero, strictly based on Newtonian mechanics.
I said it in two examples, both correct. One case is where the fall time is exactly equal is when the difference is less than 10-43
second. The other is when the objects are falling under equal gravitational acceleration. (Newtonian mechanics itself is only an approximation of reality, so if you're going to complain about an experiment that deviates from theory by immeasurable differences, you really shouldn't complain about relativistic or quantum effects being brought up, since those things affect the object velocities more than Newtonian effects!)
My hope is that kids in grades 6-8 are given a correct explanation for how things work - especially if they ask probing questions such as, "What if the falling object is really massive?"
When I teach, I consider the point I'm trying to teach. What the video in this APOD is trying to demonstrate is NOT that two objects of different mass dropped on the Moon land at the same time. If you walk away with that, you've missed the point completely. What it is trying to teach is an important concept, that two bodies of different mass subject to the same gravitational acceleration move at the same speed (which goes against most people's intuition). In a typical middle school classroom, if I then confounded this concept by introducing a totally different one- Newton's law of universal gravitation- many of the kids would become confused and fail to understand the key concept completely. It would be a mistake to first explain that bodies falling in a uniform gravitational field land at EXACTLY the same time (which is true), and then try to modify this by discussing an entirely different concept that they aren't ready for yet. For the purposes of demonstrating that bodies subject to equal gravitational forces experience identical motion, the Moon-feather system is a close enough approximation to make for an excellent test, without considering (in a middle school classroom) where that approximation fails.
If a kid asks "what if the object is really massive" I'd answer correctly: it makes absolutely no difference. A body of the smallest conceivable mass falling next to a body of the largest conceivable mass, in the same gravitational field, will fall at exactly the same velocity. If there's some brilliant kid in the class to asks about mutual gravitational effects and recognizes that using a large mass to produce the "fixed" gravitational acceleration isn't perfect... well, great. I'll tutor him outside the classroom. That's not an issue most of the students need to be concerned with.