Hi guys, I trying to understand one thing that I don't know where came from. To visualize the problem I will used the drawings from book. The question is for direction of Force Fb on drawing (b) and (a). I was always learned that friction is acting on direct opposite to motion. In drawing (b) we have correct direction because the wheel rotate counterclockwise so friction must act to right side to stop the motion. But we going to free body diagram in (a) and we see that friction is acting on the same direction as motion....Could someone me explain like for child why actually this happen?