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1st postulate of Special Relativity

  1. Dec 8, 2004 #1
    Hi. I will make this question small and simple, in fact, I will just give an analogy.

    According to the first postulate of Special Relativity, if I was traveling at 100 MPH, and my mass was 100 KG. I was traveling past a friend named David, who's mass is 100 KG too. Then my Kinetic Energy is 99904.5. But, according to the first postulate of Special Relativity, in my own frame of reference, my KE is 0, and David's KE is 99904.5.

    Thanks in advance, :tongue2:

    There are 3 types of knowledge in the world, knowledge, understanding, and imagination.
  2. jcsd
  3. Dec 8, 2004 #2
    I didn't see a question there so I'm guessing you're asking if your conclusion is correct. Not quite. You say "Then my Kinetic Energy is 99904.5." You gave no units so I assume this is in Joules. Kinetic energy, K, is related to rest mass m0 by [itex]K = (\gamma - 1) m_0 c^2[/itex]. Your kinetic energy as measured by you is zero since your velocity relative to you is zero. Your kinetic energy as measured by him is [itex]K = (\gamma - 1) m_0 c^2[/itex]. Kinetic energy is a relative quanity in Newtonian mechanics as well as in speciak relativity.

  4. Dec 9, 2004 #3
    yes, my main question was is kinetic energy by means of velocity relevant. I made this assumption my knowing the 1st postulate of SR. so, im correct. If I was traveling at c, my K would be 0, and all abject around me would have infite K. This is all hypothetical, but yes, my question was is kinetic energy relevant just as time is. And yes, the result of the kE was in joules. :yuck: By the way, how do you stick those fancy looking equasions into a posted message.

    Thank you

  5. Dec 9, 2004 #4


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    If you click on the fancy equation you will see the LaTeX code that he typed.
  6. Dec 9, 2004 #5
    According to special relativity as your speed approaches the speed of light your kinetic energy approaches infinity. If your speed equals the speed of light then your kinetic energy would be infinite. Since the kinetic energy must be finite it follows that your speed must be less than the speed of light. In SR it makes no sense to speak of a rest frame moving at the speed of light.

    To put fancy equations in you use latex. To see how to do this place your pointer over and click your mouse button. You will then see what text to enter to form the equation you're looking at. There is a document around here somewhere which shows you how to post things like this, i.e. it describes the syntax.

    Last edited: Dec 9, 2004
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