1. The problem statement, all variables and given/known data In the figure, a constant horizontal force Fa magnitude 10 is applied to a wheel of mass 10 kg and radius .4 m. The wheel rolls smoothly on the horizontal surface, and the acceleration of the center of mass has magnitude .6 m/s^2. Find frictional force on wheel. 2. Relevant equations 3. The attempt at a solution f = ma 10 + friction = ma = 6 friction = -4 I don't understand why my friction direction is opposite of the direction the wheel is accelerating. It should be in the same direction. Obviously the math is fine, but why is the friction going in the same direction that the wheel has the tendency to slide in?