Relativistic kinetic energy is usually derived by assuming a scalar quantity is conserved in an elastic collision thought experiment, and deriving the expression for this quantity. To me, it looks bodged because it assumes this conserved quantitiy exists in the first place, whereas I'd like a derivation based upon using KE =12mv2 in one frame, and then summing it in another frame say to get the total kinetic energy. Can this or a similar prodecure be done to get the relativistic kinetic energy?
Answer
Assuming energy conservation isn't "bodged" because at the most fundamental level, energy is defined as the quantity that is conserved as the result of the time-translational symmetry. All specific formulae for energy, such as mv2/2 in nonrelativistic mechanics, are just solutions to the problem "find a conserved quantity linked to that symmetry".
Still, you can try to achieve what you have defined. First, you must realize that K=mv2/2 only holds if v≪c: it's just not a valid formula in relativity for large velocities. It seems that you believe that E=mv2/2 is correct in some frames even in relativity but it's not. Your formula is just an approximation, via Taylor expansions, mc2√1−v2/c2=mc2+mv22+3mv48c2+…
In this SE question
How to derive addition of velocities without the Lorentz transformation?
Ron Maimon explained how velocities add. So if you want to switch to an inertial system moving by velocity v, you may calculate a rapidity from tanha=vc
No comments:
Post a Comment