1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution Working with equation 18.1 i found that However, this obviously is not the equation in 18.3. I found a source talking about this problem, and they get a similar equation http://file.scirp.org/pdf/JAMP_2014031310562629.pdf I have no clue where to go with this problem. Where is the 2 d theata / dl term coming from?