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Homework Help: Circle at speed of light

  1. Mar 16, 2015 #1
    1. The problem statement, all variables and given/known data

    A circle of radius R travels in the x direction with velocity c. The center of the circle is at -R at t = 0. a vertical line rests at x = 0. Find dr/dt, where r is the position of the intersection between the circle and the vertical line, and explain if this violates 2nd postulate of relativity.

    2. Relevant equations

    I already found
    dr/dt = c*cot(θ)

    3. The attempt at a solution

    The speed approaches infinity when θ → 0... I seriously don't know how to start / which part of relativity I have to use to solve this extreme speed... Any hint to start the problem? Thanks
  2. jcsd
  3. Mar 16, 2015 #2
    Well in fact it may not do anything because nothing is traveling faster than c, it's just the collision of the 2 lines, and nothing is wrong with that.
  4. Mar 16, 2015 #3


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    Staff: Mentor

    Right. No physical object is moving with that speed.
  5. Mar 16, 2015 #4


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    Gold Member

    That is an inadequate statement. It should be that nothing can travel as fast or faster than c, not just faster than c. (I am, of course, considering things with mass. Photons do travel at c and perhaps that's what you meant)
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