**Shear Force and Bending Moment in Cantilevers:**

In the case of cantilevers, both the shear force and bending moment are always negative. A cantilever is a structural element that is fixed at one end and free at the other end. The load applied to the free end of the cantilever causes the beam to bend downwards. This downward bending creates a negative bending moment throughout the length of the cantilever. The shear force is also negative because it is always directed upwards at the fixed end of the cantilever.

**Shear Force and Bending Moment in Simply Supported Beams:**

In simply supported beams, the shear force and bending moment can be positive or negative depending on the loading conditions. A simply supported beam is supported at both ends and is free to rotate at these points. When a load is applied to the beam, the reaction forces at the supports cause the beam to bend. The bending moment and shear force at any point along the beam depend on the magnitude and distribution of the applied load.

**Shear Force and Bending Moment in Overhanging Beams:**

In the case of overhanging beams, both the shear force and bending moment can also be positive or negative. An overhanging beam is a beam that extends beyond its supports on one or both ends. The load applied to the beam causes it to bend, and the bending moment and shear force at any point along the beam depend on the magnitude and distribution of the applied load. The signs of the shear force and bending moment can change depending on the location and direction of the applied load.

**Shear Force and Bending Moment in All Cases:**

In summary, the shear force and bending moment can be either positive or negative depending on the loading conditions and the structural configuration. While cantilevers generally have negative shear force and bending moment, simply supported beams and overhanging beams can have both positive and negative shear force and bending moment. The signs of the shear force and bending moment are determined by the equilibrium and compatibility conditions of the beam under the applied loads.