by Pete: "In practical terms, we used non-inertial frames all the time. For instance, as I'm sitting in my chair, I'm in a non-inertial reference frame. I maintain that I'm stationary in this frame, and that the pencil falling off my desk is accelerating, being pulled by gravity. In the non-inertial frame of the pencil in free fall, there is no gravitational force, and I'm the one accelerating - I'm being pushed up by the chair seat against my backside, hard enough to accelerate me at 10m/s/s." ================================================================ Yes, Pete, I do know what a non-inertial frame is and I have a general understanding of General Relativities explaination of gravity. It is not a 'force' in GR, you are accelerating against the Earth, as you say. But I haven't been able to figure out how the tides in the Earth's oceans are explained by GR. Perhaps you could give me a brief explaination, not long enough to disrupt this thread, and don't forget tides occur on both sides of the Earth at the same time. Thanks.
An inertial frame is one in which any object with no forces on it will move in a straight line at constant speed.
And a non-inertial frame, then, would simply be one in which the frame experiences a change in velocity?
Roughly speaking, yes, one_raven. A non-inertial frame is usually an accelerating frame. However, you have to be careful if you're talking about general relativity in this context, since GR says that all freely falling frames are inertial. That's why it is probably better to stick to the definition I gave previously.
Here's my understanding: If an object is in freefall, it is in an inertial reference frame. Otherwise, it is in a non-inertial reference frame. Or... In an inertial reference frame, an accelerometer will have a zero reading. I've never looked for a formal definition... perhaps I should!
WordIQ.com (1): We distinguish inertial reference frames, in which bodies maintain a uniform state of motion unless acted upon by another body, from non-inertial frames in which freely moving bodies have an acceleration deriving from the reference frame itself. WordIQ.com (2): In physics, an inertial frame of reference, or inertial frame for short (also descibed as absolute frame of reference), is a frame of reference in which the observers move without the influence of any accelerating or decelerating force. Zona Land: A non-inertial frame of reference is a frame of reference with a changing velocity. The velocity of a frame will change if the frame speeds up, or slows down, or travels in a curved path. A non-inertial frame of reference is an accelerating frame of reference. A non-inertial frame of reference is a frame of reference in which the law of inertia does not hold. A non-inertial frame of reference is a frame of reference in which Newton's laws of motion do not hold. In a non-inertial frame of reference fictitious forces arise. U.South Dakota, Modern Physics An inertial frame of reference is one in which Newton's Laws of Motion are valid. A non-inertial frame is one in which Newton's Laws of Motion are not valid.
When you say the tain is observed to accelerate, I am aassuming that accelerometers placed on the earth frame as well as the passenger train as the train pulls out of the station will indicate a positive accelerating force to the same degree seen by accelerometers placed ion the passenger train. We aer3e talking about inertial frames, where inertial is meant aws the resistence to change, as in the resistance to vary the state of a being at rest. The law of inertial governs these cases and tells us the state of rest, or uniform straight-line motion of an object will remain constant until acted upon by a force external to the object under scrutinty or observation. Special relativity is a postulate structured theory and the reason that a passenger train observes the earth frame as accelerating is because the acceleration is not a real physical process, where forces are delivered and felt by the objects undergoinf acceleration. AS acceleration is merely the second derivative of dispalcement, and a " second derivative" is a mathematical process and not a physical one means that accelerometer readings, personal sensations, direct observation of directed motion, like the 'jerking of the train' during acceleration is pureley a mathematical reality transferred by some mental dynamics based on reason, logic accumulated data are merly physical analogs being acted out for the sake of maintaining ones coherence in a world that is mathematically complex a, and subject to the necessary translation of motion by the experts who understand these sorts of things. So the second derivative of the train displacement merely describes that same force acting on the tracks. I am starting to get it, special relativity forces are delivered by pen and ink. Where contradictory experimental results indicating real track acceleration in an equivalent manner as seen in the train acceleration we are to conclude that all accelerations are equivalent. If the train speeds up so does the train station and lucky for us all the responbsibility of managing the observations and recording critical data in modern experiments is granted to SR theorists. We are told that an observer at rest with respect to his moving frame of reference may consider his motion at rest simply because he didn't know or was unable to detect that he was moving.If he looked out the window of the train and saw what we all see out the windows of trains, would this change any aspect of physical laws, or physical reality? Would the train's and train station's mutual accelerations be a physical parameter assigned to the train and station motion for all times to come? Silly little boys.
He may consider himself at rest because there is no physical difference between him being at rest or having constant velocity.
James R naturally took exception to my claim that the station didn't accelerate but only was observed to have a changing velocity. Because the station and persons standing there felt no accelerating force. James claimed that the station accelerates and disregards the physical principles of acceleration which is associated with an intendant force. But more telling in the following definition posted by Pete; which James R claims is the best one and the one he goes by, it actually seems to disagree with him on this issue. WordIQ.com (1): We distinguish inertial reference frames, in which bodies maintain a uniform state of motion unless acted upon by another body, from non-inertial frames in which freely moving bodies have an acceleration deriving from the reference frame itself.
geistkeisel: I have clearly explained the difference between an inertial and non-inertial reference frame. Your complaint about the train is that an accelerometer placed on the platform maintains a zero reading, while one on the train does not. In the train's frame of reference, the accelerometer sitting on the platform is indeed accelerating, yet it shows no reading. Why? Because all parts of it accelerate at the same rate, each experiencing a force proportional to its mass. In contrast, different parts of an accelerometer on the train experience different forces (in particular, the spring, or whatever it is which measures the internal force in the accelerometer, is stretched), and so the accelerometer on the train displays a non-zero reading. MacM: you are incorrect to claim that something I have said regarding definitions is inconsistent. If I made an error, you should be able to point it out. Please quote the contradictory statements I made, if they exist.
What you got and old BW Monitor? Please Register or Log in to view the hidden image! I highlited it in two tone color. .....from non-inertial frames in which freely moving bodies have an acceleration deriving from the reference frame itself. The station is not deriving acceleration from its FOR.
[post=701478]Here[/post] ***************Extract ***************** In a reference frame in which the tracks are stationary, the train is observed to accelerate. In a reference frame in which the train is stationary, the tracks accelerate. It is important to use the proper definition of acceleration. Acceleration is simply the second derivative of the displacement with respect to time. Therefore, in the train frame, it is not arguable that the tracks do not accelerate. The tracks are displaced over a period of time, and this occurs in such a way that the mathematically defined acceleration is non-zero in this frame. **************************** Definition: .....from non-inertial frames in which freely moving bodies have an acceleration deriving from the reference frame itself. ************************ Regarding the train you say "Observed" but regarding the tracks you claim "Actual" acceleration. That appears exactly opposite what I would accept. The train accelerates. The tracks are observed to accelerate. The tracks are not receiving any acceleration from its own referance frame. Perhaps the definiton isn't intended specifically in the way I interprete it but when you speak of non-inertial frames you are talking about accelerating frames and to be non-inertial it must have its acceleration generated from within the frame. The tracks do not and to say they accelerate is to claim they are non-inertial and I believe if this isn't defacto wrong, it most certainly is in poor usage of the word accelerate. Especially when discussing Relativity.
The words you choose make no difference, as long as you clearly indicate a reference frame. Thus, the following statements are equivalent: "According to the train, the tracks accelerate" "In the reference frame of the train, the tracks accelerate" "The train sees the tracks accelerate" "The tracks are observed to accelerate, in the train's frame" "An observer on the train sees the tracks accelerate" As a matter of fact, they are, if you want to use the definition you quoted. The train's frame is non-inertial, and that "causes" the tracks to experience an inertial force which arises only in the train's reference frame. In a sense, it arises from the chosen reference frame. As I said previously, you're mixing up the difference between accelerating and being inertial. Both the train and the tracks agree that the train's motion is non-inertial, and the track's is inertial. But they disagree on whether it is the train or track which is accelerating. Usually, this distinction is not important, but in this instance it is the crux of the matter, and is what is causing your confusion, and geistkeisel's too.
I'm not confused. I said it is a bad use of the term because to accelerate suggests a frame is non-inertial in relativity.
So Now JamesR is telling U all in this thread to consider a universal 0,0,0 as a frame of reference for a zero speed , that infact can be used against him when confronted with the RTT Relativity Experiment that he has shied away from and got inspiration of this thread. JamesR U did not answer to my Q in this thread, That’s good news for science http://www.sciforums.com/showthread.php?p=700038#post700038
