I was reading about Galileo's gravity and there I came across the formula. F= -mg. Galileo said that Force is proportional to mass but in the opposite direction. I don't want to discuss whether he was right or wrong. I just want to ask that when he replaced the proportionality sign and used a 'constant' 'g', why did he use the multiplication sign (this is the standard practice till today). In other words, whenever there is an interaction why only multiplication sign is used and not the plus (+)sign. Plus sign can also mean some kind of 'interaction'. Doesn't F= - (m+g) show some kind of interaction. I mean doesn't the + sign show interaction between two entities in nature. ( BTW, I know this formula is wrong) Forgive me, if it seems too childish a question to ask.