1. Ok, so I understand that a heavy fast mass impacting something is going to have a lot more of an impact than a light thing of the same speed, which is why person on horse going 15mph + lance > person off of horse running at 15mph + lance, like Neonivek said. That makes sense on many levels. What doesn't make sense to me is any reason given thus far about the merits of a footsoldier using a heavier weapon vs. a lighter one given all other attributes of weapon and wielder being equal (length, balance, structural integrity, etc.). As far as I can tell (purely speculation), melee weapons IRL weighed what they did because of unavoidable structural considerations and weighed as little as possible. I could be making a terrible oversimplification here, but lemme give ya something to ponder mathematically.
As armor grew heavier, so did weapons. Stronger armor requires more power, and more mass generates more power to penetrate stronger armor. Any man who is a soldier by profession works for his job. He has professional training, and does (or should do) training regularly to hone his skills. By the time he goes out into heavy combat, he would be well-prepared to do combat with the weapons and armors he were to be using. There is certainly a tipping point, however, where weight is detrimental to wielding potential, and you need to be increasingly stronger to wield a weapon effectively for a long period of time.
Weaponsmiths and armorsmiths struggled to find a perfect balance between weight, power, and structural integrity. Hence the evolution from [ copper -> brass -> iron -> steel ] over the ages. You always want the most for your money, and if you can have a sword that is as effective as your enemies, but lighter, you have an advantage. Fullers (blood grooves) were added to blades in an attempt to lighten the blade while keeping the edge of the blade as strong as possible.
Not sure how to read into what you said, so i'm taking a shot at it: Weapons were not made as light as possible. They needed to be heavy to have power. But they were made to be as light as possible so that it's wielder could still wield it. Taking into account the general force necessary to overcome armor was also necessary. More mass translates directly into more power and more force generated, and to penetrate heavier armors you needed more power & force. Power that a lighter weapons did not have.
The KE = (1/2)mv^2 (energy needed to get something up to speed) ends up being different than the energy delivered. If that were true, SCA combat does not have a min weight requirement for weapons, and everyone would have a 1lb sword.
I wish I had the schooling to explain this properly, but when swinging a weapon, energy is given to it. When it hits, most of the energy is transferred to the target. The issue becomes with the word most. If the energy to penetrate the armor is greater than the energy needed to break the weapon, the weapon will break and energy is lost. Lighter generally means weaker.
Weapons genearally were the heaviest one could use and practice with for 30 min at a stretch. Yes, sword work requires practice, lots of it. Meadeval swords practice starts with a pell, a log stuck in the ground. One would practice a sword shot by hitting the pell in the same exact spot. I would start my practice by puttin a piece of duct tape on the pell (where I wanted to practice hitting), then striking that tape untill it fell off. Somewhere between 100-300 shots daily. For 2 years. And yes, even after all this, I preferred a 4lb sword.
You sound like you do a lot of targetting practice. Good idea, I need to get in the habit of that. What I have a greater habit of doing is working on control. From some longsword training I have done, I was taught to attack a Pell, and stop your swing just before hitting the pell. However, the real trick was to use as much strength and speed as possible, while still stopping yourself. If you never hit the pell, you were never performing at the edge of ability and you would not prove. If you always hit the pell, you were being too sloppy and not learning control. The trick was to do it such that you hit the pell perhaps 2-4 times out of ten. That way you were always working on the edge of limits and always trying to improve your muscle control and strength.
Another interesting thing I think a lot of people don't consider is that armor and weapons, if you were having them made for you, were made to your own specifications. The less wealthy, the common (peasant soldiers) and some mercenaries would not have the wealth to afford custom-made weapons and would buy what they could or scavenge. But typically your equipment was made specifically to suit you. A smaller man would, typically, wield a slightly smaller blade than a larger man. Of course you need to take into account build and strength, but fact of the matter is a smaller man could not wield a larger blade effectively, not like a larger man could. Even if it doesn't come down to a matter of strength, if he has sufficient strength to wield a larger blade, it becomes an issue when it is too long for him to account for where it is, or it is too long and the blade is prone to hitting the ground and ruining his attacks or parries. General rule with the people I knew in the SCA was if you held your arms at rest and held the sword point-down, and swung it, it shouldn't hit the ground or it might be a problem. (for 1-handed swords)