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Homework Help: Speed of light dumb question

  1. Mar 14, 2005 #1
    Ok alot of people smarter than me have probable thought of this and know the flaw in my logic. However working in a guard tower in Iraq... I cant find the flaw only that I know it must be there. here is the question?
    I accelatea space ship along the x axis to 0.9 x the speed of light. then I accelarte it allong the y axis to 0.9 x the speed of light. now relitive to the x axis the ship is only travling 0.9 x the speed of light same with relitive to the y axis however along the 45 degree it is travleing aprox 1.2 times the speed of light which is impossible so where is the flaw in my logic... Thanks from a soldier who loves physics
  2. jcsd
  3. Mar 14, 2005 #2
    I think that is a good question. I had asked my high school teacher a question along the same lines when we were told that no two objects could travel the speed of light relative to each other. My question went, would not a metorite traveling 99 percent the speed of light past another metorite going 99 percent the speed of light in the opposite direction be going over the speed of light past each other?

    In truth, there is an equation to calculate that, which I looked up (from it I guessed that the numerical answer to my question was something along 99.999999 percent the speed of light). I have forgotten it, sorry.
  4. Mar 14, 2005 #3
    Thanks, but if possible can someone show me the math or a web site or keyword I can use to look it up. my mind is really craving order on this problem. Thanks again
  5. Mar 14, 2005 #4
    i'll draw a simple diagram.

    ------------------- (motion along x-axis)

    | (motion along y-axis)

    | (the way you're thinking about the problem...)

    However, mathematicians ignore - the space ship is actually travelling diagonally, if you think about it - when the ship is moving along the x-axis, and you start accelerating along y, the motion along the x still exists, and although you can consider motion in both axes as components, they don't represent the true velocity.

    and well, by einsteins relativity the object will eventually struggle greatly in its acceleration, until it can't possibly go any faster. [always <c]

    i dont know how good your maths is, but if you're still interested in this sorta stuff, have a browse on Lorentz-Transformations which show the 'correction' factors involved when it comes to near-speed of light travel.
  6. Mar 14, 2005 #5


    http://en.wikipedia.org/wiki/Lorentz_transformations (has some matrices and such)

    http://www.phy.ntnu.edu.tw/java/relativity/relativity.html [Broken] (amination example of relativity)

    just incase that isn't enough..


    Nothing is really specific to your problem, it really just hinges on simple relativity principles and components of velocity.
    Last edited by a moderator: May 1, 2017
  7. Mar 14, 2005 #6
    Thanks I understood the angluar velocity. why I asked the queston but eh stuff on Lorentz-Transformations is exactly what I was looking for. give me something good to think about on the towers... perfect
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