question_answer

  In each of the following a force, F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative, positive or zero.                                 

Answer:

                  Case I: In this case, force F is acting in the vertically upward direction. But the displacement of the object is in the horizontal direction (from west to east). The force and the displacement are perpendicular to each other. Work, W = Fs cos 90° = 0 Therefore, work done by the force is zero. Case II: Force F is acting in the direction of displacement of the car. Thus, the force and the displacement are in the same direction. W = Fs cos 0° = Fs. So the work done by the force is taken as positive. Case III: The force, F is acting opposite to the direction of the displacement, that is, the angle between the two directions is 180°. W = Fs cos 180° = - Fs So, in this case the work done by the force on the object is negative.