Sunday 27 October 2013

Statics—Forces, Subsystems,and Free Body Diagrams

The forces that we study in Mechanics of Materials generally keep the body in equilibrium, even if they also cause the body to deform .
         For this reason, Statics, which addresses the forces on bodies in equilibrium, is a critical prerequisite to Mechanics of Materials.

The central ideas of Statics are-

1.Force - A force describes the equal and opposite mechanical interaction between two bodies, one upon the other, which often are in contact. . Since a force has a magnitude, direction, and sense, we represent it mathematically by a vector. Two forces applied to a body at the same point have the same effect as their vector sum.

2.Engineering systems of interest may consist of multiple, interconnected parts, which exert forces on each other. Any pair of contacting parts can exert forces on each other. We must be prepared to consider all such forces and to quantify those deemed necessary.

3.All subsystems of a system in equilibrium are also in equilibrium. 
         A system that is at rest (or at least not accelerating) is in equilibrium.
                  Any part or “subsystem” of a system in equilibrium is also in equilibrium.The forces on the subsystem, acting in combination, keep the subsystem in equilibrium.

4.Free Body Diagram-
           A free body diagram displays all forces that affect the equilibrium of a subsystem.
In a free body diagram (FBD), we draw a subsystem and all the forces directly exerted on it by bodies external to the subsystem. The FBD is helpful because equilibrium of the subsystem is fully determined by the forces drawn in the diagram.

5.Selection of subsystems to find forces of interest -

            We can choose to focus on any subsystem. We choose particular subsystems because their FBDs
contain forces of interest that we wish to determine.
            Sometimes, we even consider a portion of a single part as a subsystem. This is important in mechanics of materials, because we often need to find the internal force that acts within a part,between one portion and another.

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