In Taylor, it is assumed that the constraint forces are perpendicular to the constraint surface. I am kind of wondering what the justification for this is. Sure, if you mention typical constraint forces (normal force, tension force), they are perpendicular to the motion of the particle. One thing I can think of is that this way, the constraint forces do no work (not sure why that would be desirable though). So, what is the motivation for this assumption in the context of Taylor?