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 None. 3. The attempt at a solution I've already seen another post on the exact same question, but I'm still unclear as to how the solution is obtained. If a forward force (force to the right, which we take to be positive) is applied at the axle (simply pushing), then, normally the wheel would just move forward without spinning... but because of friction from the surface, the wheel spins in a clockwise direction, meaning that the direction of friction is backwards (to the left). Answer should be C. If a torque is applied to the wheel, then the wheel is already spinning. Contact with the surface pushes the wheel in the forward direction. Therefore, in this case, friction is pushing the wheel in the forward direction. Answer should be B. However, the answer key states that the answer is D...? I'm wondering if the answer key, or any of my answers, are wrong. Thanks!