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Equivalence principle and mass

  1. Feb 20, 2016 #1
    Hello Forum,
    What is the whole reasoning and importance of discovering that the inertial mass, which is introduced as the resistance to change the state of motion and the gravitational mass, which promotes motion (the larger it is the larger the attractive force).

    The equivalence principle states that these two masses are equal....So? I am terribly missing the importance of this equivalence and its implications....

    Could anyone help clarify?

  2. jcsd
  3. Feb 20, 2016 #2
    The equality of inertial mass and gravitational mass suggests that an accelerating frame of reference produces effects which are exactly identical to those of gravitational attraction. That means in principal it is not possible to find any distinction between these two possibilities: (1) the reference frame is traversing a gravitational field and (2) the frame is going through any general acceleration.
    This remarkable connection (principle of Equivalence) eventually led to Einstein's General Theory of Relativity.
  4. Feb 20, 2016 #3
    Thanks AmioC.

    But didn't we already know that a gravitational attraction is a force that produces an acceleration like any other force does?

    An accelerating frame of reference is a frame that is under acceleration and the observers feels and sees force that are not real but fictitious like the centrifugal, Coriolis, etc.

    I still don't see the paramount link (which I know is there) between the two types of mass and why it sparked GR...
  5. Feb 20, 2016 #4


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    It doesn't produce an acceleration like any other force. It produces the same acceleration for all objects, like only inertial forces do.
  6. Feb 20, 2016 #5
    Newton's equivalence principle was well supported by experimental data and it guarantees that the Newtonean dynamics is full consistent with the Galilean equivalence principle.
  7. Feb 25, 2016 #6
    Thanks everyone but I am still not getting it.

    Inertial mass expresses this concept: it represents the resistance to cause a change in motion. It hinders acceleration.
    Gravitational mass expresses this concept: it represents that factor in the gravitational force that helps produce acceleration

    So the two masses try to produce very opposite effects. The two masses were measured and it turned out that the were numerical the same.

    Later on, Einstein concludes that a person inside an elevator in a uniform gravitational field and a person inside a non inertial uniformly accelerating reference frame (i.e. the accelerator itself accelerating upward) cannot really distinguish between the two situation....so?
    How does that change the way gravity is interpreted?

    In Newtonian physics we have no problem introducing non-real, fictitious forces when dealing with accelerated frames of reference to make Newton's 2nd law work out with the other real forces.

    What is so special about the way Einstein is looking at gravity? I know that in GR gravity is seen as a geometric deformation of spacetime. Mass is energy and is the cause of that deformation. Object move along "straight" lines" called geodetics.

  8. Feb 25, 2016 #7


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    But in Newtonian mechanics gravity near a big mass is not such a fictitious force. In GR it is.
  9. Feb 25, 2016 #8
    mmm...does GR look at gravity as a fictitious force? Is that Einstein's point?

    Newtonian mechanics seems to look at gravity as a real force when real small or large masses are involved.
  10. Feb 25, 2016 #9


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    The coordinate acceleration, which Newton attributes to a real force is just a coordinate effect in GR.

  11. Feb 25, 2016 #10
    Thank you!
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