1. ES2501: Statics/Unit 5-1: Free-Body Diagram
Introduction
Equilibrium Equations:
Total force is zero
Static Analysis
Total moment about
point A is zero
What are all forces acting
on the object??
Free-Body Diagram (FBD)
The first step of Static Analysis

2. ES2501: Statics/Unit 5-2: Free-Body Diagram
Basic Procedures of FBD
Step 1: Select the system (an object or a part of the object)
of interests;
Step 2: Isolate the system from its surrounding as a free-body
and replacing each of the surrounding contacting objects
by a corresponding force;
Step 3: Consider all remote forces acting on the system such as
a gravitational or magnetic force;
Step 4: Show all forces on the free-body and identify all information
regarding their magnitude and direction
Contact forces
Remote forces
“Free” Body is not free (of forces)

3. ES2501: Statics/Unit 5-3: Free-Body Diagram
General Knowledge of Common Forces:
Gravity: W=Mg, vertically downwards, acting at the center
of gravity
Spring force:
Linear spring
L0 MUST be its unstretched length
Force in a Cable:
-along the cable
-tension ONLY
-pulley changes direction
of the force

4. ES2501: Statics/Unit 5-4: Free-Body Diagram
Maximun static friction
General Knowledge of Common Forces (con’d):
Friction: tangent to the contact surface.
Constrain force:
For a smooth contact, the contact force is
perpendicular to the surface of contact.
Support Reactions:
Roller-support
Vertical motion is restricted
Horizontal motion is allowed
Assumed direction
>0 --- true direction
<0 --- true force is opposite
Pin-support
Both horizontal and vertical
motion are restricted

5. Examples: FBD of combined Pulley and cable ABCD: FBD of the pulley:
ES2501: Statics/Unit 5-5
FBD of combined
Pulley and cable ABCD:
FBD of the pulley:
Examples:
Cannot see the
internal force
Distributed force
FBD of the cable:
FBD of the block:
FBD of point D:
FBD of the spring:
Action and reaction

6. ES2501: Statics/Unit 5-6 Free-Body Diagram
Example 2:
Three FBDs can be drawn
depending on the problem
of interest
FBD of block B:
FBD of blocks A & B:
FBD of block A:
Internal forces (forces between
different parts of the system
cannot be seen in a FBD of the
System unless FBDs of the parts
are used.
Direction of force can be determined
by physical judgment. Sometimes, an
assumed direction can be used and the
true direction will be modified by the final
Answer.
Resultant forces of distributed normal pressure and
friction are used above, which is conditional depending
on the problem,