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Old Jun 21st 2009, 10:05 PM   #1
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Question Fictious forces: brain teaser

Ok so I read that the laws of classical mechanics hold in every inertial reference frame, that is, a system of coordinates that has constant velocity and direction. This is named the principle of relativity (not Einstein's). In order to understand this, I tried to think of a counter example, i.e. a scenario were the laws of mechanics would break down because the reference frame is not inertial.
Think that you are on a white universe; there is absolutely nothing, and everything is white, as in some scenes in The Matrix. Then picture a car located at a distance. It starts to accelerate towards you, so it is not an inertial frame. I thought that if you see the car as the one that is accelerating, everything is fine, since the car is in fact producing a force that accelerates it towards you. But since there is absolutely nothing else, I believe it is equally valid to view yourself as the one that is accelerating towards the stationary car, but since there is no real force that is accelerating you towards the car, then the normal laws of mechanics fall apart. In other words, the lack of distinction between who is accelerating, the car or yourself, is what causes this breakdown of the laws. This of course wouldn't happen if the reference frame was inertial, or in other words the car would be moving towards you with constant velocity.
I really want to know weather this example I formulated is valid or not. One reason that would make it invalid would be that space itself is a fixed reference frame, i.e. if you start moving then you'd be moving with respect to space itself, but this doesn't sound viable. So I now pose my first question to you:

Is space itself a reference frame, as if the white universe on which you were standing in my example was a fixed three dimensional system of coordinates?

Then I looked for non-inertial reference frames in Wikipedia and in fact it attributes a breakdown of the laws of physics in non-intertial reference frames, in the form of the generation of fictious forces. It gives the example of an accelerating car in which you are seating. Since your own body possesses inertia, it tends to remain stationary while the seat on which you are seated begins to accelerate. This makes your own body push towards the seat, opposite to the direction in which the car is accelerating, making you feel as if you were being pulled backward. This is what Wikipedia calls a fictious force, since there is no real cause of this feeling of being pushed back, except the existence of inertia. So I began to think that probably the only reason why fictious forces exist is the existence of inertia. Here is my second question:

Are fictious forces exclusively generated by the existence of inertia?

Were the answer to this question "yes", then it would mean that there is in fact a distinction between who is accelerating in my first example; it would then be the car the one that is accelerating and hence my first example would be completely ambiguous. If this were the case, then the answer to my first question would probably have to be "yes"; if its not the same to say that the car is the one that is accelerating than to say that you are the one that is accelerating, considering that you are in a completely empty universe in which only you and the car exist, then space itself is a fixed reference frame.

Probably all that I'm saying is complete nonsense, but for me to realize if it is I need those two questions to be answered, so PLEASE help me get out of the doubt and try to answer my questions.
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Old Jun 22nd 2009, 03:58 AM   #2
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Is space itself a reference frame, as if the white universe on which you were standing in my example was a fixed three dimensional system of coordinates?

A reference frame is one referring to which we can detect relative motion.

If only you and a wall were present in the universe, you can detect that there is relative motion with constant velocity between the two.But if the wall suddenly vanishes leaving only you in space, you cant detect any motion looking at space. Thus i dont thnk you can call space a reference frame.

To my knowledge fictitious forces are due to inertia ; even the coriolis force.

A friend once told me that physics is a strange subject . A centripetal force which is called real (though i cant feel it) causes the car to turn.

But the force due to which i am thrown away from the centre and can feel very well is called a fictitious / pseudo force!

Last edited by physicsquest; Jun 22nd 2009 at 09:53 PM.
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Old Jun 22nd 2009, 10:58 AM   #3
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Your argument about space not being a reference frame makes sense, so I guess I'll stick with that. I guess that indeed, inertia is the only cause for fictious forces. I'm thinking that this could be a philosophically debatable argument, since the existence of inertia is REAL and hence the so called "fictious" forces that arise from it should be real as well, even if they don't have any physical origin.
Thanks a lot!
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