[Jeff]

Bernt

You wrote-: "If B's contribution is only to turn a linear movement into a circular movement, B is not doing any work. The object is always moving perpendicular to the direction B is pushing. The angle between the object movement and B's pull force need to be different from 90* before any work is done."

I think that you are wrong. The fact that the object is changing direction means that there is a force present that is causing that change of direction. If a force is present and causing a change in the direction of the object's path, then it is doing work by moving the object in another direction. Person A represents the tangential force that causes the object to overcome friction and move forward at a finite speed. Person B is providing another force that pushes the object in a constantly changing direction at every instantaneous moment in time, and causes the object to follow a circular path. Conceptually, person B is providing a push-force that constantly deflects the object in its straight line path, so that the object is forced to follow a circular path. In a Newtonian sense, that push-force must have energy and it must be working - if it has the capacity to constantly deflect the mass of the object to an ever-changing directional path (a circular path).

I think that your idea that person B cannot be doing push-work because he is standing at 90 degrees to the circular path is incorrect. He is only standing at 90 degrees to the circular path - as result of his successful work effort of constantly deflecting the object from person's A desired straight line push-action. If person B wasn't doing any work, the path would no longer be circular and it would become a straight line path again.

You wrote-: "The object is always moving perpendicular to the direction B is pushing." That is correct, but it is a reflection of person B's effective work effort. In fact, if person B increases his push-force by working even harder, he will still always be perpendicular to the circular path, but now the circular path will have a tighter (smaller) radius. If that doesn't represent the result of increased work effort, then I do not understand simple Newtonian physics.

Jeff.