You have been hired as a manufacturing engineer and your task is to help design conveyor belt system to move parts from one part of the factory to another. Parts ranging in size from 1 kg up to 7 kg will drop onto a horizontal conveyor belt which will accelerate the parts at a=1.3 m/s^2. What is the minimum coefficient of friction between the parts and the belt to ensure none of the parts slip when they are sped up by the belt?