1. The problem statement, all variables and given/known data 2. Relevant equations Skipping this, as I just have a simple question 3. The attempt at a solution Here is the full solution, but I just have one simple question about a simple step. On the part for disk B, they establish the equation vD = vB - vD/B. In the proceeding step, they set vD = 0. I have checked out a few other solutions online and they ALL do this, but none explain. I'm curious as to why this happens. Thanks!