Free Body Diagram
Definition:
A free-body Dias diagram consists of a diagrammatic representation of a single body or subsystem of bodies isolated from its surroundings but shown under the action of forces and moments due to external actions.
For Example:
Consider a notebook lying flat on a table. The Note exists its weight on the table, and the table also exerts its own weight as well as transmits the weight of the notebook on the ground. Thus, a free body diagram for a book alone consists of its weight acting through the center of gravity and the reaction exerted on the notebook.
A free body diagram may be drawn for any single member of a system, any subsystem, or the entire system irrespective of whether the system is in equilibrium: at rest, in uniform motion, or a dynamic state of motion.
Uses of Free Body Diagrams in Statics
- The Problems involving the equilibrium of bodies under any system of forces can be simplified by drawing a free body diagram of each body separately.
- All equations of equilibrium can be applied to each free body diagram.
- The unknown forces of equilibrium of each body can be obtained very easily.
Structures and Types:
A structure may consist of a truss or a frame pin connected or rigidly secured. A truss assemblage of slender bars fastened together at their ends by smooth bolts or ball and socket joints acting as hinges. A truss, by definition, is a connected structure. Therefore, the bar members act as two force members, which can be either in tension or in compression: there can be no transverse force in a member of a truss. On the other hand, an A-frame structure consists of members subjected to a transverse load and the axial load.
A simple structure is thus a pin-connected frame or truss. A truss consists of a slender bar member who can carry no transverse loads. It follows that the loading in a truss must be at the joint only. A truss consisting of members which lie in a plane and are loaded in the same plane is called a Plane truss. If a truss is made of non-coplanar members, it is referred to as a space truss. Similarly, a frame may be a plane frame or space frame depending upon its structure.
Trusses are classified as Just Rigid, Over Rigid, and Non-Rigid Mechanism.
If the members are allowed any relative movement, then the assemblage of members is called a Non-Rigid truss or mechanism.
If the members are not allowed any relative movement, then it is called a Rigid truss.
A just-rigid truss is that which, on the removal of any single member, becomes non-rigid. Conversely, an over-rigid truss has redundant members, which may be removed to render the truss just rigid.
Some of the common assumptions that should be kept in mind:
- Each Truss is composed of rigid members lying in one plane.
- The weight of the members is neglected because they are small in comparison with the loads.
- Forces are transmitted from one member to another through smooth pins fitted in the members.
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