Question

Does friction always do negative work? If not, what's an example of friction doing positive work.

Answer 1

Friction always shows up as a force in the direction opposite
to the motion, so it always does "negative work".  That means
that friction always robs kinetic energy from the moving object.
It never adds any.

Wouldn't it be awesome if friction could do positive work !
That would mean that you could start a ball rolling around a
a circular track, and if the track was coated with just the right
kind of sandpaper, the kinetic energy of the ball would keep
increasing, as friction did work on it.  You could drain the extra
kinetic energy away from the ball, leave it enough to keep going,
and sell the extra energy.   Bada boom !  Perpetual motion !