Kinetic_Energy is already relative
wrt the initial_speed vi;
& relativistic if you (only)
swap (=reverse) its perspective
from wrt earth's minimum_speed 0
to wrt earth's maximum_speed c.
Reversing the
Kinetic_energy (perspective)
KE=m*v*va (wrt Earth’s_speed), *(-1)' gives
-KE'=m'*(-)*v'*va' (wrt light’s_speed) which
is already relativistic.
Please notice
if I let
c=v+u, u=c-v, u=-v', (prime_symbol ‘ is wrt light’s_speed) then
the (negative
Sorry, but I only live here (on Earth);
I'm going to no other planet.
So Earth was my best example.
va is relative to the initial_speed vi
which for this example
is the earth's(_speed,
which could be anything (reasonable)).
So if we are traveling
at the same speed
as the Earth
then that initial_speed vi=0
is zero
for at rest
wrt on Earth.
I agree. I only used it (=Earth) as an (easy, simple) example
to "try" & get the message across.