I Sign of the spatial part of the J.J interaction

gernos
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TL;DR
Is the ##A_\mu J^\mu## interaction energy between two separated identical current densities positive (repulsive) or negative (attractive) ?
I think it is negative because of the ##g_{\mu \nu}## metric tensor, but I am not sure.
 
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Thread 'The rocket equation, one more time'

Thread 'The rocket equation, one more time'

I already posted a similar thread a while ago, but this time I want to focus exclusively on one single point that is still not clear to me. I just came across this problem again in Modern Classical Mechanics by Helliwell and Sahakian. Their setup is exactly identical to the one that Taylor uses in Classical Mechanics: a rocket has mass m and velocity v at time t. At time ##t+\Delta t## it has (according to the textbooks) velocity ##v + \Delta v## and mass ##m+\Delta m##. Why not ##m -...
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