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Thread: Inertia in SR

  1. #1 Inertia in SR 
    Administrator KALSTER's Avatar
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    This is somewhat related to a thread in Pseudo:

    1) Let's say you have two objects. One is accelerating from the other and experiences inertia as a result. BUT how do we decide which one is accelerating? From each one's perspective the other will be moving away at an accelerating rate, but only one will experience pseudo-gravity. Though, if there is no preferred frame of reference, what decides which one should experience inertia?

    If I am not making sense, I am sorry. The same question could go for a single object in a void starting to accelerate. Why does it experience (if even it would) inertia if there is no other object or mass to move in relation to?

    I think there is a thought experiment that probes this question; the spinning bucket thought experiment, if I'm not mistaken. Can anyone explain?


    2) Let's say you have a perfect sphere with 4 perfectly symmetrical palm trees sequentially dotted 90o along the equator. The sphere is rotating at a constant rate. Will the palm trees be bent or stick up straight from the surface?

    Thanks


    Last edited by KALSTER; August 25th, 2011 at 06:48 AM.
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  3. #2  
    Moderator Moderator Janus's Avatar
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    Try looking up "Mach's Principle".


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    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  4. #3  
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    Quote Originally Posted by KALSTER View Post
    This is somewhat related to a thread in Pseudo: 1) Let's say you have two objects. One is accelerating from the other and experiences inertia as a result. BUT how do we decide which one is accelerating? From each one's perspective the other will be moving away at an accelerating rate, but only one will experience pseudo-gravity. Though, if there is no preferred frame of reference, what decides which one should experience inertia?
    Since not long after Newton, it was realized that since acceleration is relative, there really is no special inertia for moving objects. Rather, one should think of inertia as a statement about what kind of force is required to bring out acceleration relative to a given path.
    I think there is a thought experiment that probes this question; the spinning bucket thought experiment, if I'm not mistaken. Can anyone explain?
    The exception to the general rule of acceleration is rotation. Rotation around an axis is a form of absolute acceleration, though we might like to think of it as the movement of parts of an object relative to other parts. As far as I can tell, this aspect of rotation around an axis is a feature of spacetime.
    2) Let's say you have a perfect sphere with 4 perfectly symmetrical palm trees sequentially dotted 90o along the equator. The sphere is rotating at a constant rate. Will the palm trees be bent or stick up straight from the surface?
    I think this depends on a number of factors, including the atmosphere of the sphere. If the sphere were to start rotating, it might take some time for the rotation to work through the trees and the flexibility of the trees, but eventually they would stand upright. The key feature of a rotating object is its equatorial bulge.
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  5. #4  
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    1- You are using Special Relativity where you should be using General Relativity, ie accelerating frames, and yes GR is 'Machian'.

    2- What force would bend the trees back if rotation is constant ??
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