Interesting. I read sp332's original comment as having an implicit "in the absence of other external forces" (gravity accelerates things at the same rate regar

Interesting. I read sp332's original comment as having an implicit "in the absence of other external forces" (gravity accelerates things at the same rate regardless of mass) as that's often implied when generally discussing forces and acceleration. After all, it becomes hard to talk about the relationship between force and acceleration, if you can't even say that the acceleration of an object is proportional to a force acting upon it - because there might be other forces you've not taken into consideration! Yes, I won't accelerate an object if I try to push it into a barrier, because the barrier will exert an opposing force that resists my effort. Yes, an object might fall up if I "drop" it, because it's in an updraft that combined with it's drag coefficient causes more force up than gravity exerts. But springing those sorts of situations on people as "gotchas" isn't useful in a general discussion about the essence of a concept. Unless otherwise specified, assume ideal point masses in a vacuum (or whatever). If you want to add opposing forces of some kind to complicate^W make the situation more realistic later on as a more advanced topic, that's fine, so long as that's specified up-front. If complicating factors are not specified up-front, it's assumed that they're absent, because there are an infinte possible set of complicating factors which could be present but have not been mentioned yet. After all, in an atmosphere it could be accelerated at the same rate, as there's a (previously unmentioned) anemometer and computer-controlled rocket attached to the object which are set up to produce a thrust which exactly compensates for the wind resistence! So ner! :-) (And, in fact, sp332 specifically stated "even without air resistance" to explicitly constrain the conditions under which their example was valid)