PDF: http://www.aapt.org/physicsteam/2014/upload/exam1-2014-2-2-answers.pdf 1. The problem statement, all variables and given/known data A disk of moment of inertia I, mass M, and radius R has a cord wrapped around it tightly as shown in the diagram. The disk is free to slide on its side as shown in the top down view. A constant force of T is applied to the end of the cord and accelerates the disk along a frictionless surface. After the disk has accelerated some distance, determine the ratio of the translational KE to total KE of the disk, KEtranslational / KEtotal = Answer) I / (MR2 +I) 2. Relevant equations Torque = Iα = F x R KE = 1/2 mv2 or 1/2 Iω2 3. The attempt at a solution My conceptual understanding is the problem here, I think. If there is absolutely no friction, then would there be any rotation? I think there would be, but I'm not sure. Here's an attempt: KEtranslational / KEtotal = Mv2 / (Mv2 + Iω2) Substituting v = Rω, cancelling the ω2 terms = MR2 / (MR2 + I) So I get an answer close but not exactly the correct answer. The answer I got is choice E on the actual exam, meaning my attempt probably has a common mistake.