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Introduction to SR

  1. Aug 28, 2009 #1
    Hey guys can someone explain to me what does an inertial frame of refence mean? From what i know its a frame of reference whereby the observer frame of reference is at constant velocity? But i dont really get what it means.

    I have been reading some articles on SR and it seems that there is this case about a train moving at a constant speed and 2 bolts of lightning strike both end of the train at the same time. There is one observer in the train and one outside. The article claims that the observer outside the train see that the bolt of lightning strike both ends at the same time but the observer inside sees the lightning strike one end 1st. That does not make sense to me can someone explain this to me?
  2. jcsd
  3. Aug 28, 2009 #2


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    An inertial frame of reference is a frame of reference which is not externally accelerated. This is somewhat of a (typically physical) idealization, because no such thing exists in our universe (we are in the gravitational field of the earth, which rotates about its axis, and about the sun, while the solar system rotates about the center of our galaxy, etc.). But you can imagine two people in a well-shielded box, somewhere in outer space where no gravity is present. Intuitively, you probably agree that - while these two people can do an experiment to determine that they are moving with respect to each other - there is no way that they can tell which one is moving and which one is standing still (if any), without any "external" reference points. Indeed, this is precisely one of the postulates of relativity.

    In more practical terms, suppose that I am in a train which moves at constant velocity and you are in another train which is standing still. When I look out the window, and our trains are very close together so I can't see any "external" reference points such as the platform or the station building, I may be inclined to think that I am the one standing still, and the other train is actually moving; while you may think that you are standing still and I am moving. (This actually happened to me more than once while I was in a train). In fact, without looking at something outside the trains, there is no experiment we could do to determine which of us were moving... or maybe both.

    About the thought experiment with the lightning bolt, I think you got that backwards. One other postulate of SR is that light (in vacuum) always moves at the same velocity to any (inertial) observer. So when I am in the middle of the train, and the lightning hits both ends simultaneously, it will have to travel half the distance of the train from both ends. Since that takes the same time, I will see them arrive simultaneously. For someone outside the train, however, this is not the case. While the flash from the rear of the train moves towards the middle, the middle is moving forward. So the distance the light has to travel increases. Similarly, the middle of the train is moving towards the position where the front of the train was when the lightning struck, so the front flash has to travel less distance to me. So when you are on the platform watching me, and also see both flashes of light moving at constant velocity, the one at the back will arrive at me later then the one on the front, simply because it has to cover more distance.
  4. Aug 28, 2009 #3
    Alright thanks i think i understood them now. But one more question, why is taking an inertial frame of reference of important in SR? Is the reason just because the theory states that all frame of reference must give the same result? So what happen if i dont consider an inertial frame?
  5. Aug 28, 2009 #4


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    An inertial reference frame is one in which Newton's First Law works. That is, if the net external force acting on an object is zero, then the object either remains stationary or moves in a straight line with constant speed.

    In a non-inertial reference frame, objects accelerate "all by themselves" even when the net force on them is zero. You have to either (a) change the laws of motion, for example by modifying Newton's First Law to say that if the net force is zero, an object has a certain "intrinsic acceleration"; or (b) invent "fictitious forces" that produce this acceleration and which have to be included in Newton's First Law.
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