ME 221 Statics
Lecture #22
Sections 5.1 – 5.4
ME 221
Lecture 22
1
Homework #8
• Chapter 9 problems:
– 42, 43, 50 & 55
• Chapter 5 problems:
– 11, 13, 16, 20, 24 & 25
– See Blackboard for additional information
• Due Wednesday, October 30
ME 221
Lecture 22
2
Quiz #5
• Friday, March 14
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Lecture 22
3
Chapter 5
Equilibrium of Rigid Bodies
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Lecture 22
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Equilibrium of Rigid Bodies
• Equilibrium equations
• Free body diagrams
• Modeling supports
• Problems
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Lecture 22
5
Equilibrium of Rigid Bodies
•Newton’s Second law states that if there is a
net force acting on the body, then this will
cause motion of the rigid body.
•If there is no motion, then the object is said to
be in equilibrium.
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6
Equilibrium Equations
When the force system is replaced by a resultant force and
moment that are zero, the rigid body is in equilibrium.
F  0
~
and
M  0
~
The moment equation is new and differentiates particle from
rigid body equilibrium.
F
F
F
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x
0
y
0
z
0
M
M
M
Lecture 22
x
0
y
0
z
0
7
Supports for Rigid Bodies
If a rigid object is subjected to some set of forces
but does not move, then its motion could be
restrained by a normal force exerted by the ground,
a wall or from fixing the object with some support.
Examples of supports:
• rollers
• cables
• smooth surfaces • links
• rockers
• fixed
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Lecture 22
• clamps
• slots
• collars
8
Support Reactions
• If the support prevents translation in a given
direction, then a force is developed on the member
in that direction. Likewise, if a rotation is
prevented, then a couple moment is exerted on the
member.
• See Figures 5.1 and 5.2 (supports for rigid
bodies subjected to 2-D and 3-D force systems)
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Lecture 22
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Free Body Diagram
• Draw the body separate from all other bodies
(including ground).
• Draw the magnitudes and directions of all external
forces acting on the body.
– Include: applied loads, reactions due to supports, and
the weight of the object.
– No need to scale arrow size
• Include necessary dimensions of the body
– Dimensions are needed for summing moments
• Draw the positive sense of the coordinate system
used to write out equilibrium equations
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Importance of FBD
• The FBD is at least half of an
equilibrium problem.
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Chapter 5 Equilibrium Examples
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