Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Inertial reference frame

  1. Jan 14, 2013 #1
    hi guys, i have a basic question on special relativity.. if the inertial reference frame denotes the frames that holds up the Newtonian 1st law, then can earth be an inertial frame?
    i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??
    plz clear my concept..
  2. jcsd
  3. Jan 14, 2013 #2
    You can imagine an inertial reference frame on earth with earth's velocity being tangent to it's curved path.
    but this frame would only be applicable at one point in spacetime.

    Don't take my word for it though, I've only just recently started looking into physics :).
  4. Jan 14, 2013 #3


    User Avatar
    Science Advisor

    If you fix your reference frame to Earth then strictly it isn't an inertial one but you can take it to be an approximate one.
  5. Jan 14, 2013 #4


    User Avatar
    Science Advisor
    Gold Member

    And for those of us riding on the surface of the earth not near one of its poles, our direction and magnitude of velocity go through a cycle of change daily. In fact it was this characteristic that Michelson and Morley were hoping to capitalize on to measure a daily fluctuation in the ether wind, but since they couldn't, they concluded that the earth must be dragging the ether along with it. Of course, other explanations prevailed.
  6. Jan 14, 2013 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Newtonian mechanics and GR have different definitions of an inertial frame. This frame is noninertial according to the Newtonian definition, but inertial according to the relativistic one.

    http://www.lightandmatter.com/html_books/genrel/ch01/ch01.html#Section1.5 [Broken]

    You asked about the frame of the orbiting earth, i.e., a frame tied to the earth's center of mass. A frame tied to Los Angeles is noninertial according to both definitions, because of the earth's rotation about its own axis.
    Last edited by a moderator: May 6, 2017
  7. Jan 15, 2013 #6
    thanks to all of you...
  8. Jan 15, 2013 #7


    Staff: Mentor

    In relativity in the presence of gravity there are no global inertial frames, only local ones. In other words, you can make a reference frame where Newton's laws hold to within any arbitrary experimental precision by making your frame sufficiently small in both space and time that tidal effects cannot be measured.

    One other thing to note is that local inertial frames in relativity are in free-fall. A frame at rest on the surface of the earth is accelerating upwards.
  9. Jan 15, 2013 #8
    As others already mentioned, the earth can only be approximately an inertial frame for certain "local" experiments. That is also the case in classical (Newtonian) mechanics, so it's nothing "new".

    Very clearly, concerning special relativity:
    "this theory asserts only the equivalence of all Galilean (unaccelerated) coordinate systems".
    - https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

    See also:
    - SR uses the reference systems of Newtonian mechanics. http://www.fourmilab.ch/etexts/einstein/specrel/www/
    - Definition of inertial frames for classical mechanics and SR in: Fundamental University Physics (Mechanics), Alonso&Finn
    (question: which textbooks do you use?)
    Last edited: Jan 15, 2013
  10. Jan 15, 2013 #9
    To be precise, they were hoping to at least capitalize on the seasonal velocity differences of the earth (from orbital motion) - which is exactly what the OP mentioned. See:

    Note that I agree with your description of inertial reference frames in the other thread:
    Last edited: Jan 15, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook