ENGI 1313 Mechanics I
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Transcript of ENGI 1313 Mechanics I
Shawn Kenny, Ph.D., P.Eng.Assistant ProfessorFaculty of Engineering and Applied ScienceMemorial University of [email protected]
ENGI 1313 Mechanics I
Lecture 23: Equilibrium of a Rigid Body
2 ENGI 1313 Statics I – Lecture 23© 2007 S. Kenny, Ph.D., P.Eng.
Mid-Term
Thursday October 18 Material: Chapter 1 to 4.5 inclusive Time: 830am-945am Location: EN 2043, EN 1040, EN 2007,
EN 1001, EN 1003 & EN 1054• Seating arrangements
http://www.engr.mun.ca/undergrad/schedule.php
3 ENGI 1313 Statics I – Lecture 23© 2007 S. Kenny, Ph.D., P.Eng.
Quiz #4
Week of October 22-26 Section 4.6 through 4.10
Excluding “Reduction to a Wrench”
4 ENGI 1313 Statics I – Lecture 23© 2007 S. Kenny, Ph.D., P.Eng.
Chapter 5 Objectives
to develop the equations of equilibrium for a rigid body
to introduce the concept of the free-body diagram for a rigid body
to show how to solve rigid body equilibrium problems using the equations of equilibrium
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Lecture 23 Objectives
to identify support reactions to establish the free-body diagram for a
rigid body in 2-D to develop the equations of equilibrium for
a 2-D rigid body
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Recall – Particle Equilibrium (L10)
Concurrent Force Systems
F1
F2
F3
V = 0, v+Y
+X
0Fx
0Fy
2 Equations Solve for at most 2 Unknowns
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Rigid Body Equilibrium
Forces are Typically not Concurrent Potential moment or couple moment
0Fx
0Fy
0Mo
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Recall – Particle FBD (L10)
+Y
+X
= 30
FAB
W = FAC = mg
FAD
W = (255 kg)(9.806m/s2) = 2.5kN
A
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Rigid Body FBD
What is it? Sketch or diagram illustrating all external
force and couple vectors acting on a rigid body or group of rigid bodies (system)
Purpose? A visual aid in developing equilibrium
equation of motion
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Rigid Body FBD (cont.)
What is the procedure? Draw isolated or “free” outlined shape
• Establish idealized model• Establish FBD
Show all forces and couple moments• External applied loads• Rigid body self-weight• Support reactions
Characterize each force and couple• Magnitude• Sense • Direction
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Rigid Body FBD (cont.)
Drilling Rig 200 kg suspended platform on derrick tower
Drill Rig Idealized Model Rigid Body FBD
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Rigid Body FBD (cont.)
Cantilever Beam 100 kg beam
Idealized Model Rigid Body FBD
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Support Reactions
Newton’s 3rd Law External loads Support specific characteristics Translation prevented
support reaction force Rotation prevented
support couple moment
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Common Structural Supports
Cable
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Common Structural Supports (cont.)
Roller
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Common Structural Supports (cont.)
Pin
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Common Structural Supports (cont.)
Fixed
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Example 23-01
Foot Pedal FBD Spring force is 30 lb
Foot Pedal
Idealized Model Rigid Body FBD
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Example 23-02
Dump Truck FBD 5000 lb dumpster supported by a pin at A and
the hydraulic cylinder BC (short link)
G
W = 5000 lb
2030
B
FCB
Ay
Ax
Rigid Body FBD
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Comprehension Quiz 23-01
Internal forces are _________ shown on the free body diagram of a whole body.
A) always B) often C) rarely D) never
Answer: D
21 ENGI 1313 Statics I – Lecture 23© 2007 S. Kenny, Ph.D., P.Eng.
Comprehension Quiz 23-02 The beam and the cable (with a frictionless
pulley at D) support an 80 kg load at C. In a FBD of only the beam, there are how many unknowns?
A) 2 forces and 1 couple moment B) 3 forces and 1 couple moment C) 3 forces D) 4 forces
Answer: C Ay
Ax
FBD
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Example 23-03
Draw the free-body diagram of the beam supported at A by a fixed support and at B by a roller. Explain the significance of each force on the diagram.
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Example 23-03 (cont.)w 40
lb
ft
a 3 ft
b 4 ft
30 deg
Ax, Ay, MA effect of wall on beam.
NB force of roller on beam.
wa
2 resultant force of distributed load on beam.
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Example 23-04 Draw the free-body
diagram of the automobile, which is being towed at constant velocity up the incline using the cable at C. The automobile has a mass M and center of mass at G. The tires are free to roll. Explain the significance of each force on the diagram.
Given:
M 5 Mg d 1.50 m
a 0.3 m e 0.6 m
b 0.75 m 1 20 deg
c 1 m 2 30 deg
g 9.81m
s2
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Example 23-04 (cont.)
Given:
M 5 Mg d 1.50 m
a 0.3 m e 0.6 m
b 0.75 m 1 20 deg
c 1 m 2 30 deg
g 9.81m
s2
NA, NB force of road on car.
F force of cable on car.Mg force of gravity on car.
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Textbook
Problems
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Textbook
Problems
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Textbook
Problems
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Textbook
Problems
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References
Hibbeler (2007) http://wps.prenhall.com/
esm_hibbeler_engmech_1