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Thread: Tangential Speed Question

  1. #1 Tangential Speed Question 
    Forum Radioactive Isotope zinjanthropos's Avatar
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    2 questions...Let's say there's this huge spinning disc with a circumference of 299,000km (slightly less than the distance light travels in 1 second). The rotational speed is 1 revolution per second. Would that mean the rotational speed cannot be increased because a point on the rim cannot go faster than c? Along the same lines....since all points closer to the centre are travelling shorter distances with lower tangential speeds, would their highest speed be limited (never get as close to c as the outer edge) because the outer edge points can't exceed c?


    Last edited by zinjanthropos; October 31st, 2018 at 11:10 PM.
    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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  3. #2  
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    I didn't see two questions. However, the speed limit (c) applied to the out edge will limit the rotation speed.


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  4. #3  
    Forum Radioactive Isotope zinjanthropos's Avatar
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    Quote Originally Posted by mathman View Post
    I didn't see two questions. However, the speed limit (c) applied to the out edge will limit the rotation speed.
    Apologies mathman....I meant to delete that first sentence after I realized I'd asked the same question twice....my bad.

    Don't know why I find that answer interesting. It is what I figured it would be. Question came about when I had this thought of a spinning disc universe.

    Is there a physical law that can state the exact velocity of a point anywhere on the spinning disc between centre and rim if the velocity of the outer edge is known?
    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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  5. #4  
    Moderator Moderator Janus's Avatar
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    Here's the problem. Relativity doesn't allow for perfectly rigid objects, so your disk can't be perfectly rigid. This means that it will deform from the forces acting on it. Long before the rim ever reaches close to c, it will tear itself to pieces.
    A spinning disk universe would not be anything like a rigid object. There is nothing that would make all the different parts of it maintain the the same angular speed.
    Objects would tend to want to migrate from the axis to the rim, but as they did so they would maintain their tangential velocity and thus decrease their angular velocity.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  6. #5  
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    Consider an infinitely strong and mechanically rigid disk (thought experiment) one inch in circumference. You apply 1 hp energy to the axis at 2 rps friction free. You expand the disk to a larger circumference. As the disk expands the edge accelerates and continues to draw energy to accelerate the disk. This is kinetic energy. As the edge nears the speed of light its rotations per second reduces but the axis rps doesn't. At a large enough circumference the axis energy of 2 rps doesn't accelerate the edge rps any more. So where's the energy going? It's not turning into kinetic energy (acceleration). It's turning into relativistic mass. Physically what's happening to the disk? Seems to be twisting in a spiral. The axis is turning faster than the edge after all. It's not a mechanical spiral though. It's a relativistic spiral of the reference frame.
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