4 ms·

> tells the students that gravity accelerates things at the same rate regardless of mass Please get your wording right, there is a big difference between force

• sp332 13y ago
  You have it backwards, qwerta. If the feather and the hammer fall to the ground in the same time they must have accelerated at the same rate. Of course the hammer has more force applied by gravity and the feather has less; that is why the hammer is heavier!
  • qwerta 13y ago
    Yes, but in atmosphere it would be accelerated at a different rate. Stationary objects are not accelerated at all, because gravitational force is compensated. As I was saying, there is difference between force and acceleration.
    • Karellen 13y ago
      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)
      • qwerta 13y ago
        This is not about edge conditions, but about two different concepts. Saying "gravity accelerates" does not make sense by definition, at least we should say "gravitational force accelerates".
        • deleted 13y ago
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    • Double_Cast 13y ago
      sp332: > gravity accelerates things The confusion is between the transitive and intransitive forms of the word "acceleration". If I say "the pen accelerates", this implies net acceleration of the object. If I say "gravity accelerates the pen", this implies gravity's contribution towards net acceleration. Air resistance diminishes net acceleration, but not the acceleration as contributed by gravity. sp332 was fine. It's common for astrophysicists to talk about gravitational fields in terms of "acceleration fields" rather than "force fields". This is because the acceleration contributed by gravity only depends on a single argument (distance from the celestial body's center) as opposed to a force-field's two arguments (distance, the pen's mass). And next time you hear "g = 9.81 m/s^2 for all objects on earth's surface", the physicist is referring to gravitational acceleration, regardless of whether or not the object is moving.