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2 questions

  1. Feb 27, 2008 #1
    Hi, I accidentally stumbled upon this forum while looking for something else. Seeing that there are a lot of people here who really know what they're talking about, I'm going to ask 2 questions that have been "haunting" me for quite some time.

    First question:
    If you would drill a hole from one side of a planet (with no core) to the other, and if you would drop an object through that hole; would that object come uit at the other end, or would it simply get stuck in the middle?

    My second question involves breaking the speed of light.
    If you would let a train ride in a circle, with both ends touching eachother, and you would place tracks on that train, and let another train ride on it; would it not be possible to break the speed of light without using an infinite amount of energy?

    Like if you would let 11 trains driving at 1/10th the speed of light, drive on each other, would it not seem for someone standing at the bottom that the top train is breaking the speed of light?

    (of course you would have to place something around the trains to keep them in track)

    thanks in advance for answering my question
  2. jcsd
  3. Feb 27, 2008 #2


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    1.) Ideally it would alternate in much the same manner as a damped harmonic oscillator and eventually come to rest at the dead centre (where there is no field). Or maybe it'll oscillate slightly in the centre. For all intents and purposes, it will be near the centre.

    2.) That's the kind of thing special relativity is exactly against. Velocity addition in SR is much different from Newtonian physics. Can't remember the math bethind it but no, your trains will not exceed the speed of light.
  4. Feb 27, 2008 #3


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    Gold Member

    Although it's not scientific, I find that the easiest way to think of the second question is that 1+1 always equals something a little less than 2 when adding speed. It's totally insignificant under any real-world circumstances, but becomes quite pronounced at relativistic speeds.
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