Constant velocity: find the friction force. In this constant velocity concept question, we are given a block moving at constant velocity, and we want to find the friction force at constant velocity. We don't have any coefficient of friction to find the friction force: find the friction force using Newton's second law and the fact that the acceleration is zero! To get the constant velocity friction force, we use the face that the net force on the block is zero, so the friction force balances applied force. The friction force has the same magnitude as the applied force, so the forces add to zero.