Jewish_Vulcan said:
the answer is: the force in the horizontal direction has a net of 0, when they say for a certain distance like 10m they make you "assume" that the person came to a stop after the 10 m, for him to come to a stop an equal but opposite force must act against the motion. because w=fd, w=0(10)=0,
I'm not at all clear what you are saying here, and maybe others have misinterpreted it.
First, there's no quoted context, so I don't know whether this is the inclined plane model or the horizontal plane model.
Re " when they say for a certain distance like 10m ", it would help if you were to give a complete example of such problem.
I'll suppose you mean something like "A man weighing 700N walks on a horizontal path for 10m in 15 seconds. How much work does the man do?"
I would first criticize the problem statement for not specifying whether the man was already walking before the 10m started, nor whether, as you say, he came to a stop at the end of the 10m.
Suppose it's rest to rest. As mentioned by others, even human gait (quite efficient really) is not 100% efficient, so it takes work just to walk at a steady pace. Leaving that aside, he has to accelerate from rest (steadily for 5m, say), then decelerate to a halt. During the deceleration, work is done on the man, technically, but the man is not able to turn that back into useful energy. It all ends as heat, which the man needs to lose. So whether the net work is zero or positive depends whether you wish to include the heat output as negative work for the man. The word 'work' takes on different meanings in different technical contexts.
Mark44 said:
A simpler example that might be easier to understand involves a smooth ball rolling on a flat, frictionless surface
Umm... if it's rolling, why do you care about friction?