A force can retard the motion of an object only when it will acts in opposite direction of motion of body, this means that angle between the direction of motion of body and direction of force is 180°.

When force is applied at an angle to the direction of motion of object then the work in the direction of motion is given by: W = Fs cos θ

Since cos 180°= -1,

So, Work done = -F×s

which is negative.