Hi. A friend of mine asked me an interesting question a couple of days ago and I've been thinking for several days but I cannot find a logical answer since I'm not taking any physics course in university. I typed physics forum in google to find the logic of the question and found here. The question is sth like that: We know the direction of friction force is opposite to direction of motion but let's assume just a wheel moving to right-side in a 2D world. We're expected that the direction of friction force is left-side but it didn't work like that. At the point where wheel and ground touch each other, the wheel seems like moving left; so the direction of friction force will be right-side. So it means the friction force is seen as a positive force for motion, since the direction of wheel's motion and the direction of friction force are both right-side. How could it be possible? Finally, sorry if I wrote a word or a sentence mistake. English is not my native.