Equilibrium of rigid bodies: Part 1

Static equilibrium:

A body or part of it (shown in Fig. 1) which is currently stationary or moving with a constant velocity will remain in its status, if the resultant force and resultant moment are zero for all the forces and couples applied on it. So:

Free body diagram for rigid bodies:

In order to draw the FBD for each member of a rigid body follow the instructions below:

• Isolate the object from its surroundings,
• Draw the outline of the object; consider all dimensions and angles,
• Include all forces and couple moments that the surroundings exert on the body. Forces include loadings, support reactions and weights. (See the support reaction section for detailed explanation)
• Known forces and moments should be labeled with their proper magnitudes and directions.
• Magnitudes and direction angles of unknown forces and moments should be represented with letters.

Example:

Draw the FBD for the following beam: A pin can be considered for support A , A roller can be considered for support B, Weight of the beam is generally neglected when it is small compared to the load the beam supports (or has not been mentioned).

The FBD is: