Question

Is it possible that an obj is in acc motion by external force but force does no work?

Answer 1

Yes , It's possible only when the force applied at 90° on the object... Here's the proof..

Let Force F acting on a object at 90° and displacement be D..

So ,
W = FD Cos theta
W = FD Cos 90°
W = FD × 0 ( since Cos 90° = 0)
W = 0