Free Body Diagrams
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Understanding Free Body Diagrams: Components, Procedure, and Practice, Summaries of Acting
Learn about Free Body Diagrams (FBDs), a graphical representation of forces acting on an object. FBD components, procedure, and practice with examples and different types of support reactions. Students can use this as study notes, summaries, or cheat sheets for mechanics courses.
Typology: Summaries
2021/2022
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Free Body Diagrams
Free Body Diagram
A Free Body Diagram is a visual representation of force and object interactions.
Individual objects or members are isolated from their environment or system, illustrating all external forces acting upon them.
Free Body Diagram Components
The twisting effort (force) about a point or
axis when a force is applied at a distance.
Drawn as an arc with an arrowhead acting about
a point indicating direction of CW or CCW.
Moment
Distance (d) is called the moment arm (or lever
arm). It must be measured perpendicular to
the line of action of the force.
Moment (Torque) Review
Line of Action F
Point of Rotation d
Moment (M) = Force (F) x distance (d)
Free Body Diagram Procedure
1. Sketch the isolated object.
What is the isolated object? Top Book
Free Body Diagram Procedure
2. Sketch the applied and
normal forces.
When an object is in contact with and is supported by a second object, the second object can be replaced with a normal force which is perpendicular to the surface of the second object.
Free Body Diagram Procedure
3. Label objects and forces.
N=5 lbf
W=5 lbf
PLTW – DE book
Free Body Diagram Procedure
4. Label dimensions.
N=5 lbf
W=5 lbf
PLTW – DE book
For more complex free body diagrams, proper dimensioning is required, including length, height, and angles.
45 °
8 ft
10 ft^ 8 ft
38.6°
Free Body Diagram Practice
W (^) W θ
θ
Create a FBD for the refrigerator pictured below.
Free Body Diagram Practice
M
M
FBD of Mass 1:
FT FBD of the movable pulley:
W 1
W2 + W pulley
FT
FT
Tension Forces (FT ) are equal throughout the system.
Create a FBD for the pulley system pictured below.
Cable, rope, chain – Replace with a
tension force only.
Cable Support
A sign with weight W is hung by two
cables as shown. Draw the FBD of the
sign and cables.
Cable Support
Pin – Replaced with TWO reaction forces,
one vertical (y) and one horizontal (x).
Pin Support
A (^) RAx
RAy
Reaction Force
Joint / Pin A
y-direction
Reaction Force
Joint / Pin A
x-direction A
Roller – Replaced with ONE reaction
force, perpendicular to surface.
Roller Support
A A
RAy^ y-direction
Reaction Force
Joint / Roller A