1. The problem statement, all variables and given/known data A man is walking in a straight line in a constant speed V. All variables are known. Since the speed in constant, the net force is zero. But aside from the static friction which is allowing him to move forward, what other force is present (in the direction of the movement), which is supposed to cancel out the friction so that the net forces is zero? 2. Relevant equations [tex]\sum[/tex]F = 0 3. The attempt at a solution I tried comparing it to walking on a treadmill on a constant speed. In this situation, there is no (or isn't supposed to be) static friction, since you're not really pushing yourself forward, only "jumping" while the belt is going backwards, so that you stay at the same position. But when walking on the ground, I have to use friction in order to move forward, so what is the force that is opposite in direction to the static friction that is acting upon me? If this is supposed to be in "Classical Mechanics" forum, sorry..please move the thread.