Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy in different references

  1. Jan 4, 2010 #1
    I was thinking in special relativity and how does the energy look from one reference to another,
    if i consider a body making a circular motion in reference R1, with an angular velocity w1,and a stationary body in reference R2, R1 see that the body in R2 is rotating with an angular velocity |w1|, so the body in reference R2has a kinetic energy 1/2(I*W1^2) from reference R2point of view.
    But the idea which is puzzling me is: Is this kinetic energy really exist?!
  2. jcsd
  3. Jan 4, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi hisham.i! :smile:

    Are you using the rotating body in R1 as a reference frame?

    You can't do that in special relativity …

    all frames must have uniform velocity and be non-rotational. :wink:
  4. Jan 4, 2010 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Tiny-tim, that's not correct. You can use any coordinate system you want as long as you don't call it an inertial frame.

    hisham.i, your question doesn't have anything to do with relativity. You have the same issue in Newtonian mechanics. The answer is yes, the kinetic energy is different in different coordinate systems.
  5. Jan 4, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper

    Hi Fredrik! :smile:
    Yes, I agree, but I meant frames "in special relativity", by which I took hisham.i to mean frames related by the Lorentz transformation.
  6. Jan 4, 2010 #5
    Yes you are right..
    but the point is can i really use this energy in the other reference i mean can i generate electricity from this rotation?
  7. Jan 5, 2010 #6
    yes, everything has energy so far it once had a rest mass..or relativistic mass
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook