A spring with k=45 N/m is .35m when pulled down with a 1.0 kg mass, what is the length of the spring when the mass is taken off? The answer in the back of the book is .13m (13 cm), I can't for the life of me figure out how they get that. I know that Hooke's Law is: F = -kx What I've done so far: F = (45)(.35) = 15.75 ---> Force it takes to pull down .35 meters, apparently using 1.0kg. Thus, if I divide 15.75 by 9.81, I should get 1.0kg, yes? 15.75 / 9.81 = 1.60 ---> I must be doing something wrong, lets start backwards. F = (45)(.13) = 5.85 --> Force it takes to pull down .13 meters (this x is the answer, but right now i'm trying to find a proper F). 5.85 / 9.81 = .56, again, not 1.0kg. :( What am I doing wrong? Can anybody point me in the right direction?