15 Year Member
Quick answer is you’ve confused energy with momentum. (And kinetic energy is 0.5 x mass x velocity squared)Mass x Velocity SQUARED.
Regardless, people don't watch gladiator sport to see how gently the players treat each other.
Hits are why I fell in love with football. The violence is why most people watch. The violence is why they are compensated the way they are. If they were worried about their future, they probably shouldn't be playing football.
Mass times velocity is momentum. And change in momentum equals Force imposed on body. And and in a collision it dictates which body has the greatest change in velocity. The bigger discrepancies in size these days along with greater velocities means the smaller guys take More of the force than in past.
It’s like two vehicles collide head on, both going 60 mph and one is 2000 lb and the other 4000 lb. change in momentum of each body is equal to force and the small one is toast.
From that you can calculate the change in kinetic energy in each; it will probably be the little guy there too, especially if he was moving fast.
Simple answer (as it’s 6 am and I don’t want to spend more time thinking) is both equations apply. Forces will be equal and energy of system unchanged.