20.1104 Intro to Engineering Analysis – Studio Version
Fall 1997
A Guide to Drawing Free-Body Diagrams
Purpose: Up to this point in IEA there has been some confusion as to what exactly constitutes a free-body
diagram (FBD). In general there is a certain amount of variation in the definition depending on which
resource you are using. Some resources have a very strict definition of what should be included on a FBD.
One example of this is the textbook, Engineering Mechanics: Statics by Riley and Sturges. Others may
have a somewhat simpler, but in many senses, equally applicable definition of a FBD. This document will
explore the differences among the various types of FBDs and will discuss the relevance of each. Each
method will be illustrated through the use of an example problem. The original problem is shown below
and the subsequent FBDs for each method will then be presented following a description of each method.
The uniform 10-ft boom CA weighs 350 lb. And is supported by a ball and socket joint at A and two cables
attached at B and C. The boom and cable are in a horizontal plane. Determine the tension in the two cables.
Method 1: Straight and Narrow
Riley and Sturges outline a method for constructing FBDs that is very complete. FBDs which result from
their method generally contain all of the details given in a problem, such that the final FBD could be given
to a complete stranger, who could then solve the problem having never seen the original diagram of the
problem. The method is outlined below.





Decide which body or combination of bodies is of interest for a given problem
Draw a simple outline of the body of interest, isolating it from its constraints
Represent all forces and moments that act on the body as vectors on the diagram
Choose and label a set of coordinate axes
Place any necessary dimensions and angles on the diagram
20.1104 Intro to Engineering Analysis – Studio Version
Fall 1997
Example: FBD Constructed Using Method 1
Notes:
1) Boom has been isolated.
2) All forces are represented, i.e. two
tensions, the weight and the three
reactions at A.
3) Coordinate system with labels.
4) Dimensions shown.
5) The sense of tension TCD is clearly
shown using its intercept.
6) Forces are labeled as vectors.
7) Guaranteed to receive maximum
credit on test.
Method 2: FBD as a Visual Tool
This method has been used occasionally in class. For this method, the FBDs are similar to those of the first
method but now the emphasis has been taken away from the details and placed on the overall appearance of
the FBD as an adequate representation of the problem. Using this method one can clearly see what forces
are acting on the body. It is important to have the following elements.





Isolate the body of interest from its constraints
Place all forces and moments acting on the body on the diagram
Label the non reaction forces and moments as vectors. Reaction forces can be shown as the scalar
magnitude of the force, since the direction is already assumed to be along one of the coordinate axes
Show and label a set of coordinate axes on the diagram
Give some indication of important angles, but do not dwell on all of the given dimensions
Example: FBD constructed according to method 2
Notes:
1) Boom has been isolated.
2) All forces are represented.
3) Coordinate system with
labels.
4) Dimensions not shown.
5) Weight and tensions shown
as vectors.
6) Reactions shown as scalar
components; directions are
along x, y, and z.
7) Adequate for TEST, likely
to receive full credit.
20.1104 Intro to Engineering Analysis – Studio Version
Fall 1997
Example: Poorly Constructed FBD
Notes:
1) Boom has been isolated.
2) Weight has been forgotten.
3) No coordinate axes shown.
4) Tension, TCD, is not labeled.
5) Tension, B, is not shown as a vector
(no bar shown).
6) Minimal credit on a TEST.