# Dynamics in two dimentions

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1. Mar 26, 2015

### aracali

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!

Last edited by a moderator: May 7, 2017
2. Mar 26, 2015

### Staff: Mentor

Why don't you present your math here, step by step? If you also include the constants that you use (there aren't that many!) you can avoid the image links.