1. The problem statement, all variables and given/known data During paddling of a bicycle, the force of friction exerted by the ground on the two wheels is such that it acts (A) Front Wheel -----> Backward | Rear Wheel ------> Forward (B) Front Wheel -----> Forward | Rear Wheel ------> Backward (C) Front and Rear Wheels -----> Backward (D) Front and Rear Wheels -----> Forward 2. Relevant equations Pure thinking 3. The attempt at a solution I thought it like this. We have a bicycle and it is moving towards +ve x-axis. Now at the bottom part of the wheel(which is in contact with the road and where friction is acting) is moving clockwise. The bottom part will then have a motion towards left and in order to avoid slipping, friction must act in the forward direction. As in the case of rear wheels, friction should act in the backward direction in order to help the motion of the bicycle to move towards +ve x-axis. But my answer is not correct. Its says just opposite to my thinking.