FREE BODY DIAGRAM

 Free-body Diagram (FBD)

The isolation of a mechanical system is achieved by cutting and isolating the system from its surroundings.  The isolation enables us to see the interactions between the isolated part and the other parts.   The part which has been cut (imaginarily), forms a free body.  A diagram which portrays the free body, complete with the system of external forces acting on it due to its interaction with the parts which have been removed, is called the free-body diagram (FBD) of the isolated part.

The FBD of of a body system shows all loads acting on the external boundary of the isolated body.  The loads include active forces applied on the free body by the parts removed from it.  Note that the internal forces acting on a structure becomes external forces when exposed by the imaginery “cutting” process carried out to form the free body.

As an example, consider the structure of the arm of a lift truck, Figure 3.1.  Assume that an analysis is to be carried-out on the whole structure of the arm when it is carrying a load as shown, where the weight of the component members of the arm can be nglected compared to the weight of the load. Assume also that all joints of the arm do not prevent rotation around the respective joints, i.e. every joint produces a reactive force only and does not produce any reactive moment.  Because the direction and the sense of every reactive force are not known, the direction nad sense shall be assumed.

The arm can be isolated from the body of the lift truck at point A where it is pinned to the body of the lorry and at point C where it is acted upon by the active forceof the hydraulic piston rod.  The isolated arm is shown in Figure 3.1(b).  On the diagram, FA is the force produced by the interaction between point A on the arm and that on lorry body, FC is the interactive force between point C on the lorry arm and the hydraulic piston, mg is the weight of the load, and  G is the centre of gravity of the load.  Figure 3.1(b) is the FBD for the entire arm for the conditions specified.

If what is to be analysed is the load container only, the FBD shown in Figure 3.1(c) is drawn.  If member BC is what one wants to analyse, the FBD is shown in Figure 3.1(d).

Please note that, in the FBDs shown, the direction and sense of all the reactions are drawn arbitarily because they are assumed to be unknown.  We will learn later how to determine the direction of some types of reactivce forces through observation.