I'm having difficulty understanding why the radius is not represented by r, but Lcostheta , isn't the entire rope in the horizontal direction? There is something I'm missing. Thanks. (Hopefully I posted this correctly.) 1. The problem statement, all variables and given/known data The problem is example 1 on http://physics.bu.edu/~duffy/py105.html You have to click on Circular Motion on the left-hand side of the site. 2. Relevant equations F=mv^2/r 3. The attempt at a solution It has already been solved on the web site.