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Achieving speed of light via rotation on an axle?

  1. Aug 31, 2011 #1
    I was reading about dentist drills, and I discovered an amazing piece of trivia about them: modern dentist drills spin at 400,000 revolutions per minute (over 6666 times per second!).

    So I thought, if this was spinning a large wheel, whose outer rim would rotate at a rate progressively faster than the inner rims, could it ever achieve the speed of light? Technically, I know that the speed of light (c) is a physical limit, but what would happen to such a physical device?

    So you have an axle run by a powerful motor that rotates the wheel it is attached to at a rate of 6666 revolutions per second (speed of dentist drill).

    Speed of light (c) is ~300,000,000 meters per second. At 6666rpm, to achieve the speed of light we would need a wheel whose circumference is (300,000,000 / 6666) 45,000 meters, which would equate to a diameter of (45,000 / pi) 14,324 meters - roughly 9 miles.

    So we create a structurally-sound, 9-mile-wide wheel and rotate it on an axle, slowly but surely until we achieve 6666rpm.

    To run the motor would require an almost unimaginable amount of energy (I reckon), and to find enough material to create a wheel that is structurally sound enough to not fall apart in the process would most likely be inconceivable, but ...

    If we could capture enough energy, and obtain the necessary materials, could this work? If not, why not? Something tells me that the laws of physics somehow prevent the physical integrity necessary for such a wheel, but this is simply a guess.

  2. jcsd
  3. Aug 31, 2011 #2


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    Staff: Mentor

    Well...if you understand that C is a physical limit (and you understand why?), why would it matter if you attempted to reach it with linear or rotational acceleration?
  4. Aug 31, 2011 #3
    Because you will need an infinite energy... and there's no "enough" energy that is infinite!

    The point is that as the speed of your wheel (or anything else) approaches C, to bring this speed to C you will need E->infinite. So, if you are a genius with a lot of money buy your wheel or create it, but don't say I Had not told you!

  5. Aug 31, 2011 #4
    Greetings Andrew,

    Even if it were possible to provide enough energy to force a large wheel to achieve a tangential velocity of light-speed, c, (299,792,458 m/s) and assuming light-speed were actually physically achievable; the G-force experienced by the wheel would be colossal and cause it to fly apart long before you could achieve a velocity that approached a miniscule fraction of c.

    The acceleration (a) experienced at a given tangential velocity (v) by whatever mass per the radius (r) of the wheel can be derived by the following kinematics equation:

    a = (v)^2 / r

    Let’s assume it’s a huge wheel with a radius (r) of 100 meters and we would like to spin it at a velocity (v) of c:

    (299,792,458 m/s)^2 / 100 meters = 8.987551787e+14 m/s^2

    As you can see, the acceleration (a) is nothing short of colossal and nothing could withstand the G-force per any building materials known by man.

    I hope you found this helpful.
  6. Sep 1, 2011 #5
    Whether you're accelerating something tangentially or linearly, as it's velocity (linear or tangential) approaches light speed, it's mass will still begin to approach infinity. And even if it didn't, like Gnosis said, we don't have any materials that wouldn't simply be ripped apart by the magnitude of force necessary to spin something that fast.
  7. Sep 1, 2011 #6
    I posed this question many years ago on a filesharing forum I used to frequent (for a long time I refused to believe that nothing could travel faster than light) convinced I had artfully smashed the cosmic speed limit via a crude thought experiment involving a huge pole stuck to a spinning wheel or something. A wise (if a little sarcastic) fellow forum goer simply linked me something along the lines of this http://www.youtube.com/watch?v=FmRV8wNJdKY&feature=related

    not massively helpful I know, but I enjoyed the visual aid.
  8. Sep 1, 2011 #7


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    Staff Emeritus
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    2018 Award

    Lol that video was awesome!
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