The work done by a constant force is equal to the integral of force with respect to displacement along the path of the displacement. Depending on the direction of force applied with respect to displacement, work done can be positive, negative, or zero. For instance, when a person pushes a box on a slanting plane, the force applied and the displacement are acting in the same direction. So, the work done by the applied force is positive. When the same block slides back, the applied force is acting opposite to the displacement. Hence, the work done is negative. Consider a person carrying a box and moving horizontally, here the work done by the person is zero since the angle between the force and displacement vector is 90 degrees. The work done is maximum when the angle between the force and displacement vector is zero or 180 degrees.