- #1

polytheneman

- 6

- 3

**constraint forces do no work because they are perpendicular to the allowed virtual displacements of the system.**However, if you consider an unbalanced Atwood machine, in which both masses are accelerating in opposite directions, you'll find that

**the tension force of the wire (a constraint force), which pulls the lighter mass up, is parallel to the displacement, which means it does work (right?).**

Now, I understand that

**the same is true for the other side**: the tension force on the heavier mass is parallel to the displacement, but in the opposite direction, so that

**if you add the work done by the tension force on the heavier side to the work done by the tension force on the lighter side you get zero.**

So my question is: would it be correct for me to say that

**individual constraint forces can do work, but it's the sum of the work done by all the constraint forces which is always equal to zero?**If this is true, it's a bit different from the notion I had before, which was that individual constraint forces never do work because they are always perpendicular to the displacement.