Mass dilation would of course be the relativistic mass (a term which really shouldn't be used.) It seems none of the books or websites I have researched have explained this to my satisfaction, so I began thinking about this, and here is my derivation. NOTE: IRF = Inertial Reference Frame What do you all think? The key points are that the time used for the derivative is the proper time of the object, but the length is the observed length (which would be contracted) of the object - and that any force being applied to an object would be observe to be the same from any IRF - i.e., force is invariant. Alternatively, if anyone knows of a very clear discussion somewhere, I'd be all ears & eyes!