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Homework Help: Special relativity from different angle

  1. Jul 18, 2006 #1
    If we accelerate one body with mass m0 on a distance dx, then, we need energy dW:
    dW = a m0 dx = dv/dt m dx = m0 dv v
    Energy dW, which we gives weight dW= dm c^2
    If we put this in differential equation, solvation is
    dm/m0 = v dv /c^2
    But this calculation does not give that m is infinity at v = c.

    What failing to my calculation?

    (I know normal derivation of special relativity equation)
  2. jcsd
  3. Jul 18, 2006 #2


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    Staff Emeritus
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    You are assuming that in SR, F=ma, where m is the 'relativistic mass'.

    This formula is fundmanetally wrong. See for instance


    You can also take a look at http://www.math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

    in the addendum "what is the relativistic version of F=ma"
  4. Jul 25, 2006 #3
    I found another formula for relativistic mass which works:
    c^2 dm = m v dv gama^-2

    The result is m = gama m0
    Where m0 is mass at v=0.
    This give the same formula of relativistic mass in longitudinal direction as in horizontal direction. This gives hint that absolute explanation is possible. This means slower time-run in rocket which moves very fast is consequence of larger mass in rocket and so means that we see mans in rocket moving slow. But how to explain that peoples in rocket see us that our clock run slow?

    I understand that two main postulates of relativity,
    1.speed of light is the same in every inertial system
    2. All system are equally valid.
    But one explanation should to exist, if we suppose that our system is privileged. Suppositions are more involved than with points 1 and 2, but some explanation should to exist.
  5. Jul 27, 2006 #4
    Above mentioned relativistic mass (longitudinal = transverzal) is useful, because it is alternative explanation for time dilatation:
    Because mass is larger, everything is moving slower and this gives slower time running.
    But how this concept explain that someone in moving rocket see time dilatation on earth?
    I suppose that that classical special relativity is only the most simple view on special relativity, but it is not the only one.

    I think that this question of beginners, but thread was moved.
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