Not a criticism Shazam.. but don't you mean it the other way around? Weight is based upon the gravitic pull of the largest body acting on it. But the Mass never changes... that's a constant; and my point. "Magic" might be able to make the weapon feel weightless.. but it still retains its mass.. hence why getting hit with it would be fatally painful... for the few second one was still alive after such a devastating impact.
The magic does not really make the weapon inertialess... or it would have absolutely NO impact no matter how fast you swung it on contact.. its 'inertia-less'. So the magic is twisting gravity fields in some way to make it 'seem' weightless but retain its mass.
My point is if Bern's swords negate that magic~ when he swinging the I-Beam... at that speed the inertia of the mass of the I-Beam is WAAY HIGH.. and his tightly gripping hand will feel all that tidal effect of the inertia he has imparted to it with his swing! Momentum is a bitch.. it will likely rip his fingers off if not pulverize every bone in his hands, as the grip violently flies out of his hands from the momentum of the swing... flesh & bone have their shear force limits... like everything else made of matter.
I pick up a bullet and throw it at you and it bounces off your shirt... no biggie, it's not very heavy at all. Now I do it again.. but this time I add several thousands of meters of velocity on it and it blows a hole through your chest and the brick wall behind you! It didn't get heavier.. but its inertia made the impact Much more devastating. Now do that same with the I-Beam. Its already damned heavy... so getting hit with it will... hurt {to say the least}. But, try holding onto it with it moving several hundred feet per second while your standing still... Good-bye hand! 'nuff said.
Nope.
Sorry for the delay in replying.
Weight is solely your angular momentum based on a force (in this case gravity) being applied to a mass. Ie, gravity as a force measures the pull and thus acceleration on an object towards the source of the gravity.
I'd been pondering how to simplify things via text (before I forgot about this thread). Reducing an item's weight, simply means you are reducing how much force you need to lift it vertically. That's all. You are simply addressing it's relatively momentum downward. It has no other meaning of effect. It doesn't allow you to increase your acceleration of an object for a given net force. That would require changing the mass.
To give another mind experiment. If you have ever seen the challenges of someone pulling a large mass, perhaps a car, or a sled with weights on it, they are having to strain against two elements, the first is the inertia of the object (ie, it is at rest) and in addition whatever friction is inherent in the system... is in wheels, flat against the ground, etc. The most difficult stage is the beginning and starting to get the mass moving. They have to add enough force to the system to accelerate it and change it's momentum. Once it is moving, they can keep pulling it more easily. It will tend to keep moving only shedding speed based on energy lost to friction. An object at rest tends to stay at rest, and an object in motion tends to stay in motion.
The short answer is, whatever he's doing it isn't changing 'weight'.
A solution to what he is doing is that he's transferring the effect mass of the system that is him plus his ibeam to his body and thus changing his the rotational angular momentum of the system. Ie, like when a ballerina/figure skater pulls their arms/legs in after starting to spin which dramatically accelerates the spinning.
http://physics.stackexchange.com/questions/3611/why-does-a-ballerina-speed-up-when-she-pulls-in-her-armsThe 'magic' is that the ibeam isn't actually pulled in, so the location of impact will be on the end portion of the weapon which is now moving ridiculously fast, and with kinetic energy being 1/2 mv^2, you get his legendary (unbeknownst to Polly apparently) destructive capability.
So his magic is a faking out of the rules of physics by shifting mass within a system. Ta da.