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Wrong signs for Lorentz velocity problem

  1. Sep 17, 2014 #1
    1. The problem statement, all variables and given/known data

    A neutral K meson at rest decays into two pi mesons, which travel in opposite directions along the x axis with speeds of 0.828c. If instead the K meson were moving in the positive x direction with a velocity 0.486c, what would be the velocities of the two pi mesons?

    2. Relevant equations

    v'x = (vx - u)/(1-vxu/c2)

    3. The attempt at a solution

    I reasoned that one meson velocity can be found be plugging directly into the above formula with vx = 0.828c, and u = 0.486c. As for the other meson, I reason that the velocity vx will be the negative of the vx from the first meson, while u is unchanged because the K meson is moving the same direction relative to me for both mesons.

    I am getting the negative of the correct answer. Is it equivalent (I am standing on "opposite sides of the situation") or did I make a sign error/ false assumption? Thanks a ton in advance,

    Last edited: Sep 17, 2014
  2. jcsd
  3. Sep 18, 2014 #2


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    Gold Member

    Its OK. Anyway the two mesons are moving in opposite directions and so if you take one meson's velocity to be positive, the other one's would be negative. And even if it wasn't the case, that negative sign is only about the direction of the motion.
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