Under what circumstances ( if any ) is no work done on a moving object even though a net force acts upon it?

Answer 1

When an object is moving in uniform circular motion, a net force acts upon the object (centripetal force), however, no work is done on the object since the net force is directed perpendicular to the direction of motion.

One definition of work is a Force applied in the direction of motion over a distance. Centripetal force is always pointed towards the center of rotation, but at any instant in time, the object's velocity is pointed tangent to the circle along which it is moving, thus no work in done on the object.

Answer 2

A rock thrown from a space shuttle. SInce there is no resistance force in the vacuum of outer space, the rock must continue forward (Newton's First Law of Motion), though, in that case, there is no net force...

Answer 3

When an object is in a vaccum, or a blackhole.