If I put a 1 kg mass on the floor, the floor exerts a 10 N force on it, balancing gravity. If I then replace that mass with a 10 kg one of the same area, the floor now exerts 100 N on it. What is it that makes it exert different forces? I know of Newton's third law of opposite and equal reactions, but that doesn't seem to explain it, it just seems to say 'because it does'. Also, I know that the electrons in the floor and the electrons in the object are repelling each other, but why does the upward force vary?