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Need help understanding inertial frames of reference

  1. May 4, 2012 #1
    Need help understanding inertial frames of reference!!

    I'm doing an A2 physics unit on special relativity (AQA) and am really confused about this, but I only want to get the idea so don't go to deep please :)

    I understand that a frame of reference is an area which is fixed relative to something (e.g. Our frame of reference is fixed relative to us). In my textbook it says that an inertial frames are frames of reference that are moving at a constant velocity relative to each other (not accelerating). My teacher (who's crap so I don't trust what he says) says that if there are two frames moving at constant velocity relative to each other, each of those frames are inertial frames themselves, but the whole thing overall (including both frames) is not inertial. So is the whole thing- including both frames- an inertial frame, or is each individual frame an inertial frame?

    My revision book says that an inertial frame is one which Newton's first law applies...is this a definition, or just a way to test if something is an inertial frame? Also it gives an example of an accelerating train with a ball in it, the ball will move relative to the train without a force being applied, so Newton's first law is broken therefore it is a non-inertial frame...but is the train accelerating relative to something (another frame, e.g. the platform)? If so, how come the accelerating train is not just an inertial frame from the frame of reference of an observer on the platform?

    One more thing (sorryyy)...one of Einsteins postulates is 'The laws of physics have the same form in all inertial frames'. What does that actually mean?

    I hope I've explained what I mean okay...thanks :)
  2. jcsd
  3. May 4, 2012 #2


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    Welcome to PF!

    Hi BomboshMan! Welcome to PF! :smile:
    Same thing. :wink:
    No, a frame of reference is a coordinate system, a grid of imaginary lines drawn in space.

    It's your coordinate system if you think the grid is stationary. :wink:
    This only works if you already know that one of them is inertial …

    once you check Newton's first law for one frame, you then know that any frame moving uniformly wrt that frame is also inertial.
    That makes no sense … "the whole thing" is two grids passing through each other. :frown:
    It gives you a familiar example so that you know exactly what it means.

    "An accelerating train" means exactly what a normal person would describe as an accelerating train.
    The same equations work for any inertial observer.
  4. May 4, 2012 #3


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    Re: Need help understanding inertial frames of reference!!

    Here's a concrete example of what it means. Suppose you have two physics laboratories, each equipped with whatever equipment you like, except that nothing can "see" through the walls of the labs, and there are no windows, etc. All experiments you can do are confined inside one lab or the other.

    If both labs are inertial frames (each lab is stationary in some inertial frame), then there is no experiment that you can do (inside either lab) to decide whether either of them is moving or not.

    For example, one lab might be fastened to the ground (and we ignore effects due to the earth's rotation, which can be detected e.g. with a Foucault pendulum), and the other might be in the back of a truck/lorry/whatever with a perfectly smooth suspension, traveling down a perfectly smooth straight level road at 100 km/h (62 mi/h). Or one might be in an airplane with silent engines, flying in a straight line through still air at 500 km/h. All the laws of physics work the same way inside both labs.
  5. May 4, 2012 #4
    Re: Need help understanding inertial frames of reference!!

    Thanks for the replies :D

    So...if you perform exactly the same experiment in tow inertial frames you will get the same results, no matter how fast they are moving relative to each other...and I guess this is why it's impossible detect absolute motion?

    With a non-inertial frame (one that is accelerating), surely it's accelerating relative to something else? If so, you could just as well say that 'something else' is the one accelerating relative to it, which would make it an inertial frame...or is acceleration somehow not relative to anything?
  6. May 4, 2012 #5


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    yes :smile:
    it's accelerating relative to almost everything else!

    so what?

    the test is Newton's first law
  7. May 4, 2012 #6
    Re: Need help understanding inertial frames of reference!!


    Except only in one of the frames will there be a feeling of force – the one we refer to as ‘accelerating’. The other ‘non-accelerating’ frame feels nothing. The astronauts in a spacecraft accelerating away from Earth feel ‘g-forces’ during the period of acceleration. If they’d left some colleagues back orbiting the Earth, the colleagues would have felt absolutely nothing.

    So, unlike the situation where people in two different inertial frames can’t say which one of them is ‘really’ moving (because the situation is completely symmetric), the case where one frame is accelerating is asymmetric.
  8. May 4, 2012 #7
    Re: Need help understanding inertial frames of reference!!

    Okay, I think I've finally got it :D...thanks for the help :)
  9. May 5, 2012 #8
    Re: Need help understanding inertial frames of reference!!

    "am really confused about this,.."
    yup...most people start out that way because it requires some difference ways of thinking.....and some careful personal thought....

    To expand just a bit on:

    yes. What this means is that if you did not already know one was inertial, you could have two equally accelerating frames moving along with constant velocity between them. Think of them as starting from rest, for example, and accelerating equally in parallel paths.....if they were two rocketships, passengers in each would look out and see the other ship 'stationary' alongside...but each is actually accelerating.
  10. May 5, 2012 #9
    Re: Need help understanding inertial frames of reference!!

    In my own thoughts I "define" an inertial frame as one in which an observer can release an object in front of his eyes, and the object does not move away. Therefore I don't worry too much about circular definitions. On the other hand, I was downvoted on Reddit for suggesting such an interpretation, was/am I wrong?
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