1. The problem statement, all variables and given/known data http://damtp.cam.ac.uk/user/dt281/dynamics/two.pdf" [Broken] Looking at page 5, equations (2.19) and (2.20) 3. The attempt at a solution I cannot understand how they derived the (2.20), at first from comparing the solutions I had assumed r(dot)' had disappeared as we were differentiating with respect to dr'. I then went about the derivative of the cross product: Omega X r(dot)' + .... But in the solution we find Omega x (Omega X r'). Could anyone please help clear this up for me.