# Inertial reference frame

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..
thanks

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 :).

WannabeNewton
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.

ghwellsjr
Gold Member
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..
thanks
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.

bcrowell
Staff Emeritus
Gold Member
i mean it changes direction of velocity as it moves in the orbit around sun, so its not in constant velocity ??

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.

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thanks to all of you...

Dale
Mentor
2021 Award
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..
thanks
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.

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..
thanks
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".

- 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?)

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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.
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:
https://en.wikisource.org/wiki/The_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether
https://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

Note that I agree with your description of inertial reference frames in the other thread:
https://www.physicsforums.com/showpost.php?p=4228938&postcount=62

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