Once again, chinglu demonstrates that he doesn't know what he's talking about.
If you have two frames L, L' and L' is in motion, L stationary, this is exactly equivalent to the converse: L is in motion and L' is stationary. Mixing frames means you do something like assume x in frame L is the same variable as x' in L', when obviously, it isn't, because L isn't equivalent (or equal) to L', even if their origins coincide (because, to paraphrase Einstein, that's only possible for an imaginary interval of time, so that simultaneity is us fooling ourselves that time really exists, we can synchronise clocks--but "when" do we do this??).
The reason for the above is simple: motion is relative. But (caveat!) it only applies to uniform (i.e. constant) motion and hence inertial frames.
If you have two frames L, L' and L' is in motion, L stationary, this is exactly equivalent to the converse: L is in motion and L' is stationary. Mixing frames means you do something like assume x in frame L is the same variable as x' in L', when obviously, it isn't, because L isn't equivalent (or equal) to L', even if their origins coincide (because, to paraphrase Einstein, that's only possible for an imaginary interval of time, so that simultaneity is us fooling ourselves that time really exists, we can synchronise clocks--but "when" do we do this??).
The reason for the above is simple: motion is relative. But (caveat!) it only applies to uniform (i.e. constant) motion and hence inertial frames.