1. The problem statement, all variables and given/known data A 60.0 kg woman stands at the western rim of a horizontal turntable having a moment of inertia of 500 and radius 0f 2.00 m. Turntable is initially at rest and is free to rotate around frictionless vertical axle through its center. Woman then starts walking around the rim at the constant speed of 1.50 m/s relative to the earth. OK. Is mechanical energy of the system conserved? 2. Relevant equations Conservation of mechanical energy 3. The attempt at a solution OK. So my woman here is exerting a force pushing on the disk and the disk is pushing a force on the woman. These are equal and opposite forces.... and are internal to the system. I said that mechanical energy is conserved as there are only forces internal to the system. My solutions manual says the following, however: "The mechanical energy of this system is not conserved because the internal forces, of the woman pushing backward on the turntable and of the turntable pushing forwards on the woman, both do positive work, converting chemical to kinetic energy." Huh? The two forces are internal... and thus do not affect the mechanical energy.... or do they? Who is right: the solutions manual, or I? If the solutions manual is right... then why is their approach right with the term "positive work?" Thanks in advance.