8 ms·

He's not entirely wrong. The thing with centrifugal force is that by picking a different frame of reference, it completely disappears and the reason it arises

He's not entirely wrong. The thing with centrifugal force is that by picking a different frame of reference, it completely disappears and the reason it arises is due to your frame of reference being different. Interestingly, however, is that it is possible to pick a frame of reference such that gravity disappears, so it too is a fictitious force in a way.

danbruc 12y ago
Interestingly, however, is that it is possible to pick a frame of reference such that gravity disappears, so it too is a fictitious force in a way. I am not a physicist but that seems wrong to me or at least imprecise. You can select a frame of reference that makes gravitational forces disappear locally but there is no frame of reference that makes them go away globally.
- jacquesm 12y ago
  That's the whole point of the exercise: to show that these frames of reference are all relative. And if you think about it, the exact point of origin of the 'big bang' might actually be a frame of reference relative to which gravity disappears globally. Relativity is a very tricky concept. Of course gravity is a 'real' force but the ability to shift your frame of reference is a powerful reminder that gravity and acceleration can look the same until you exit your frame of reference.
  - danbruc 12y ago
    I am not sure if I just misunderstand you, but we don't know if there was something like a singularity at the big bang and I would guess many doubt that there was. But if there was such a thing it was probably not at a specific point but everywhere. And you don't have to leave your frame of reference to see a difference between acceleration and gravity, they are distinguishable in any frame of reference.
    - SixSigma 12y ago
      You're also talking as though the "big bang" is proven.
      - danbruc 12y ago
        Depends on how narrow or broad you interpret the term. I would say back to the origin of the cosmic microwave background 380,000 years past the big bang the story is pretty well understood. Back to ten to the minus thirty two seconds the model still looks reasonable to me. The time before that makes me feel uneasy, inflation looks to ad hoc to me as a laymen. And I really don't believe there was some kind of singularity with infinite density at the very beginning, but I doubt many do. You can of course never prove something in science but the model seems to provide a reasonable explanation of our observations. There are of course definitely issues and things not yet understood and other things may turn out to be incorrect but that is the nature of science.
    - stolio 12y ago
      > And you don't have to leave your frame of reference to see a difference between acceleration and gravity, they are distinguishable in any frame of reference. One of Einstein's great contributions is the knowledge that this isn't actually true. If you're in a box with no windows it's literally impossible to tell if you're on the ground in a planet's gravity or if you're being accelerated in empty space. http://en.wikipedia.org/wiki/Equivalence_principle http://en.wikipedia.org/wiki/Equivalence_principle
      - danbruc 12y ago
        That is actually wrong, the equivalence principle requires careful wording. Imagine a point mass. Gravitational forces will always point directly towards it and therefore the direction of the force you measure within the elevator will vary a tiny bit in order to point exactly towards the point mass. It will also vary a tiny bit in strength because of the varying distance to the point mass depending on where you measure it within the elevator. On the other hand normal acceleration of the elevator will not show any dependence on where you measure the force.
  - pavel_lishin 12y ago
    > And if you think about it, the exact point of origin of the 'big bang' might actually be a frame of reference relative to which gravity disappears globally. I thought about it - not very hard, admittedly - but I still don't see what you mean.

danbruc 12y ago

Interestingly, however, is that it is possible to pick a frame of reference such that gravity disappears, so it too is a fictitious force in a way. I am not a physicist but that seems wrong to me or at least imprecise. You can select a frame of reference that makes gravitational forces disappear locally but there is no frame of reference that makes them go away globally.

jacquesm 12y ago
That's the whole point of the exercise: to show that these frames of reference are all relative. And if you think about it, the exact point of origin of the 'big bang' might actually be a frame of reference relative to which gravity disappears globally. Relativity is a very tricky concept. Of course gravity is a 'real' force but the ability to shift your frame of reference is a powerful reminder that gravity and acceleration can look the same until you exit your frame of reference.
- danbruc 12y ago
  I am not sure if I just misunderstand you, but we don't know if there was something like a singularity at the big bang and I would guess many doubt that there was. But if there was such a thing it was probably not at a specific point but everywhere. And you don't have to leave your frame of reference to see a difference between acceleration and gravity, they are distinguishable in any frame of reference.
  - SixSigma 12y ago
    You're also talking as though the "big bang" is proven.
    - danbruc 12y ago
      Depends on how narrow or broad you interpret the term. I would say back to the origin of the cosmic microwave background 380,000 years past the big bang the story is pretty well understood. Back to ten to the minus thirty two seconds the model still looks reasonable to me. The time before that makes me feel uneasy, inflation looks to ad hoc to me as a laymen. And I really don't believe there was some kind of singularity with infinite density at the very beginning, but I doubt many do. You can of course never prove something in science but the model seems to provide a reasonable explanation of our observations. There are of course definitely issues and things not yet understood and other things may turn out to be incorrect but that is the nature of science.
  - stolio 12y ago
    > And you don't have to leave your frame of reference to see a difference between acceleration and gravity, they are distinguishable in any frame of reference. One of Einstein's great contributions is the knowledge that this isn't actually true. If you're in a box with no windows it's literally impossible to tell if you're on the ground in a planet's gravity or if you're being accelerated in empty space. http://en.wikipedia.org/wiki/Equivalence_principle http://en.wikipedia.org/wiki/Equivalence_principle
    - danbruc 12y ago
      That is actually wrong, the equivalence principle requires careful wording. Imagine a point mass. Gravitational forces will always point directly towards it and therefore the direction of the force you measure within the elevator will vary a tiny bit in order to point exactly towards the point mass. It will also vary a tiny bit in strength because of the varying distance to the point mass depending on where you measure it within the elevator. On the other hand normal acceleration of the elevator will not show any dependence on where you measure the force.
- pavel_lishin 12y ago
  > And if you think about it, the exact point of origin of the 'big bang' might actually be a frame of reference relative to which gravity disappears globally. I thought about it - not very hard, admittedly - but I still don't see what you mean.