1. The problem statement, all variables and given/known data A forward force on the axle accelerates a rolling wheel on a horizontal surface. If the wheel does not slide the frictional force of the surface on the wheel is: A. zero B. in the forward direction C. in the backward direction D. in the upward direction E. in the downward direction 2. Relevant equations N/A 3. The attempt at a solution The answer key says that the answer is D. My textbook shows a diagram indicating that the answer is B. A similar question online says that answer should be "in the backwards direction (and does zero work on the wheel)." Which one is it? I'm inclined to go with B because that's what the diagram in my textbook looks like (ie. on a horizontal surface, if the wheel is accelerating forward, static friction acts forward as well at the bottom of the wheel). But then there's the case where the wheel is rolling down an incline. Then the acceleration is down the incline but the static frictional force is up the incline which is different from the previous situation. Maybe I don't understand this concept very well, can someone explain?