Question

when is a workdone by a force said to be negative

Answer 1

hi there,
work done by a force is said to be negative when the displacement is against the directing of the force acting on the object.in this case the angle b/w displacement and force is 18 degree. 
i hope this helps u...

Answer 2

work done is said to be negative when displacement is in opposite direction to the force applied . 
 i hope that this will help u ...........................^_^