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Homework Help: Two SR Questions

  1. Jul 29, 2008 #1
    1. The problem statement, all variables and given/known data

    In frame S , particle 1 is at rest and particle 2 is moving to the right with velocity v.
    Consider frame S ′ , moving relative to S , with speed u. Find the value of u such that
    the two particles appear in S ′ to be approaching each other with equal but opposite

    I don't see here how you can get the particles to approach each other.

    1. The problem statement, all variables and given/known data

    Calculate the range of speeds for a particle of mass m in which the classical relation
    for kinetic energy is within 1% of the correct relativistic value.

    For this can I use K/T = 0.01 and solve for the velocity of the mass, where K is newtonian KE and T is relativistic KE?
  2. jcsd
  3. Jul 29, 2008 #2
    What equations do you think you should use on the first question? You're kind of close on the second part, but you want the difference in K and T to be 0.01 of the relativisitic..
  4. Jul 29, 2008 #3
    The velocity transforms could be used for the first question. Can I just set the S' frame velocity to -1/2v or -v (not sure here) then solve for the frame speed u given that the S frame speed is v?

    Thanks, I'll work on the second one.
  5. Jul 29, 2008 #4


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    Science Advisor
    Homework Helper

    Be careful! If v=c then there is no such frame. So the answer clearly isn't c/2. You have to use the relativistic formula for velocity addition.
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