1 Definition of free body diagram
2 Purpose of free body diagram
3 Rules of construction of free body diagram
4 External & internal forces
5 Examples of external forces
6 Application of internal forces
FREE BODY DIAGRAM
A free body diagram, sometimes
called a force diagram, is a
pictorial device, often a rough
working sketch, used by engineers
and physicists to analyze the
forces and moments acting on a
Block on a ramp (top) and
corresponding free body diagram
of just the block (bottom). For
equilibrium, the line of action of the
three force arrows must intersect at
a common point.
PURPOSE OF FREE BODY DIAGRAM:
• Drawing a free body diagram can help to determine the unknown forces
on, moments applied to, and equations of motion of the body and thus help to
analyze a problem in statics or dynamics.
• In analysis of structures, free body diagrams for a component of a structure
or, part are used in determining shear forces and bending moments
Rules to construct a valid free-body diagram (FBD):
Rule 1. Select the appropriate body or body segment, keeping in mind the unknowns you want
to compute. That is, make sure the free-body includes at least one of the unknowns.
Rule 2. Draw all known forces or moments of force at their respective points of application
(e.g., weight vector, measured external forces, etc.). Note that the weight vector is placed at
the center of gravity of the free-body.
Rule 3. Draw all unknown forces and moments that directly contact the free-body. Wherever
the free-body is separated from other parts of the body, replace the excluded parts of the
body with an unknown force (two components) and a moment of force.
Rule 4.Do NOT include Internal forces which originate and terminate within the free-body (e.g.
joint forces of internal joints).
EXTERNAL & INTERNAL FORCES:
The loads and forces which are acted on
the beam are called the external forces.
The reactions developed are also external
moment, axial force, torsional moment, etc.
which are introduced in the beam due to
the effect of external forces are internal
forces. The internal forces must balance the
external forces to satisfy the equilibrium
condition of the structure.
forces in beam
EXTERNAL & INTERNAL FORCES:
Internal loads in a structural member are the result of externally applied loads. The
external loads are transmitted to different parts of the structure through these
internal loads. The internal loads can be determined by the method of sections.
The structural action of a beam is represented by internal forces called bending
moments and shear forces. A beam is subjected to two sets of external forces.
These are the loads applied to the beam and reactions to the loads from the
supports. The beam transfers the external load set to the external reaction set by a
system of internal forces.
external and internal forces in beam
APPLICATION OF INTERNAL FORCES:
A member is cut at the point of
interest, and the internal loads are
revealed as equivalent external
loads. Since the member was in
equilibrium before being cut, these
equivalent external loads must keep
the member in equilibrium.
For the common case, there are two
forces, an axial force and a shear
force, plus one moment. These forces
and moments are shown in the
Unknown Internal Loads V, A, and M
Replace. Again, F & reactions at the
supports Ay,By & Bx are external loads.
Deleted 2D Structure Section to Maintain
As one can calculate the forces and moments transmitted through joints between
members, one can also calculate the internal forces which one part of a member exerts on
another. To calculate these internal forces, simply draw a free-body diagram of only part of
the member, cutting through the member at the point you are interested in knowing the
forces and moments. For example, consider the following member
If you are interested in knowing the forces and moments that are transmitted through the
member at point D, you can draw the free-body-diagram of the portion to the left of D to