Hi everyone, Good day. for the effect of friction on wheel (all the questions are based on rolling without sliding), i know that in the case of uniform rolling, there will be no friction on the wheel. while for accelerating or decelerating case, there will be friction on the wheel (but in certain circumstances, depends on the mass moment of inertia of the wheel, the friction can be zero). the first question is, when uniform rolling is achieved on a frictionless surface, what will happen? 1. the car will stay on the same spot with the wheel keep rolling? or 2. the car will continue to move forward with the wheel keep rolling? or 3. the car will continue to move forward with the wheel sliding? the second question is, in the case of accelerating or decelerating, does the friction act on the wheel causes loss? i mean does this friction affect fuel efficiency? the third question is, looking at the attachment, it is found that if r1 = 0.5r2, then the friction force will be zero, if r1 > 0.5r2, the friction is in the direction as drawn in fbd, if r1 < 0.5r2, the friction is opposite to the direction as drawn. what does zero friction indicates? why zero friction still allows the wheel to accelerate? other than that, what does different friction direction indicate?