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Feynman writes:

One very important feature of pseudo forces is that they are always proportional to the masses; the same is true of gravity. The possibility exists, therefore that gravity itself is a pseudo force. ... It might seem all right to consider gravity to be a pseudo force, to say that we are all held down because we are accelerating upward... Einstein found that gravity could be considered a pseudo force only at one point at a time...

Now, how can gravity be a pseudo force? What did Feynman meant by saying this statement? Is gravity really a pseudo force? If so, why?

bobie
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    The force of gravity depends on the coordinate system. In a free falling coordinate system (like aboard the ISS) there is no force of gravity, at all. This is a very important statement on which all of general relativity rests. – CuriousOne Dec 22 '14 at 16:04
  • See also Why do we still need to think of gravity as a force? – John Rennie Dec 22 '14 at 16:21
  • @John Rennie: Sir, I really do want to know why gravity is a pseudo force. But none of the questions linked do answer why gravity is a pseudo force. Really want an answer:-C –  Dec 22 '14 at 17:38
  • I wanted to know why gravitational force is a pseudo force. So,why is it duplicate?? –  Dec 22 '14 at 17:52
  • The marked duplicate why gravitation is not a force never does concern with why it is pseudo force. I wanted an answer. But it was marked duplicate with a quo that never really asked mine. And more also, there is no proper answer there. –  Dec 22 '14 at 17:56
  • @user36790 - In Newtonian physics, pseudo forces (or fictitious forces, or inertial forces) result from trying to use F=ma to explain motion from the perspective of a non-inertial frame. The same is true in general relativity. The catch: The two theories differ massively on what constitutes an inertial frame. The key difference with regard to whether gravitation is a "real" or a "fictitious" force is that a free-falling object forms the basis of a local inertial frame in GR. An object at rest on the surface of a planet? That's close to inertial in Newtonian mechanics, not at all close in GR. – David Hammen Dec 22 '14 at 19:39
  • Why the switch? First off, inertial frames are global (universal) in extent in Newtonian mechanics. That doesn't make a bit of sense in GR with its curved space-time. Secondly, and more importantly, the equivalence principle comes into play. Einstein used his elevator thought experiment as a way to elicit problems with the Newtonian concept of an inertial frame and the equivalence principle. – David Hammen Dec 22 '14 at 19:50
  • @David Hammen: Thanks for these brilliant cnmments. Then what is inertial frame in Newtonian Mechanics is not at all inertial in GR. –  Dec 23 '14 at 03:26

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