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I have had a physics problem on my mind for a few days and it seems relatively simple at first, but it has been puzzling me, and I was wondering if you could help.
Say a student rolls a tire on a flat level surface. There is no car involved or anything it’s just the tire. As the tire rolls, the tire slows down by friction and eventually comes to a stop. I have drawn an extended free body diagram below in paint.
I have the wheel rolling to the right and gradually slowing down with acceleration to the left. Since it is accelerating to the left, then that means static friction must be pointing to the left. So this is the problem: If static friction is pointing to the left, that means friction is also creating a torque on the wheel that points into the screen. If there is a torque pointing into the screen that torque should speed up the wheel’s rotation! But that doesn’t happen. So the question is: How can we use just Newtonian physics (not energy considerations) to account for the fact that the friction force is doing two seemingly contradictory things at once? It’s slowing the wheel down by pointing to the left, but also speeding the wheel up by applying a torque into the screen. I’ve been struggling with this for a few days. I was thinking maybe I’m missing a force, like air resistance. But that shouldn’t matter. We should be able to perform this same experiment in a vacuum. Somehow, there must be some torque applied out of the screen to cause the wheel to slow in its rotations. But I don’t know how.