1. The problem statement, all variables and given/known data Hello, the question itself is pretty abstract. When pulling a thread roll, there is an critical angle, where the thread does not start to unwind itself. Instead, the thread roll itself starts to move. Derive an formula for determining that critical angle. 2. Relevant equations Most probably it is about using the Newtons' first law, as the angular acceleration and speed remains constant (0). 3. The attempt at a solution I think the final formula will include centermass conception and radius of the thread-roll. But I have no idea, from where to start..