I am not sponsored by Sharpie... yet. Calculate the required coefficient of static friction required for an object to roll down a hill without slipping. The coefficient of friction must be adequate to cause a rotational acceleration that is consistent with the translational acceleration of the wheel. This solution works for any round shape of object so long as it has a consistent radius and a known moment of rotational inertia. This problem comes up in introductory physics courses including AP Physics 1, AP Physics C Mechanics and appears on the JEE as well as A Level Physics Exam.