1. The problem statement, all variables and given/known data Suppose the moon were held in its orbit not by gravity but by a massless cable attached to the center of the earth. Find the tension in the cable. 2. Relevant equations F=ma a=v^2/r v=2πr/T (since the pictures aren't working, I'm just going to post the url to the image) (http://imgur.com/Ktj98f6 [Broken]) All the numbers I used are from the back of the book, shown here. (I used the period, mass and mean radius shown in the image) 3. The attempt at a solution From all the equations, I figured that the new formula using substitution would be F=m((2πr/T)^2)/r) (http://imgur.com/e7QMdTG [Broken]) Now the answer at the back of the book says 2.01x10^20, but I keep getting 9.08x10^20. I've rechecked my math hundreds of times, and I've also tried plugging in the numbers into the original equations one at a time and I keep getting the same answer. So I figured the only thing that is wrong then is the numbers, but the numbers came straight from the book, so they can't be wrong!