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7/27/2019 Lecture 3- 1page
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© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
Vector Mechanics for Engineers: StaticsE i gh t h
E d i t i on
2 - 1
Equilibrium of a Particle
• When the resultant of all forces acting on a particle is zero, the particle isin equilibrium.
• Particle acted upon bytwo forces:
- equal magnitude
- same line of action
- opposite sense
• Particle acted upon by three or more forces:- graphical solution yields a closed
polygon
- algebraic solution
00
0
==
==
∑∑
∑
y x F F
F Rrr
• Newton’s First Law: If the resultant force on a particle is zero, the particle will
remain at rest or will continue at constant speed in a straight line.
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Vector Mechanics for Engineers: StaticsE i gh t h
E d i t i on
2 - 2
Free-Body Diagrams
Space Diagram: A sketch
showing the physical conditions
of the problem.
Free-Body Diagram: A sketch showing
only the forces on the selected
particle.
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Vector Mechanics for Engineers: StaticsE i gh t h
E d i t i on
2 - 3
EQUILIBRIUM OF PARTICLE IN 2-D
This is an example of a 2-D or
coplanar force system. If the
whole assembly is in
equilibrium, then particle A is
also in equilibrium.
To determine the tensions in
the cables for a given weight
of the engine, we need to
learn how to draw a free bodydiagram and apply equations
of equilibrium.
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Vector Mechanics for Engineers: StaticsE i gh t h
E d i t i on
2 - 4
THE WHAT, WHY AND HOW OF A
FREE BODY DIAGRAM (FBD)
Free Body Diagrams are one of the most important things for
you to know how to draw and use.
What ? - It is a drawing that shows
all external forces acting on the particle.
Why ? - It helps you write theequations of equilibrium used to
solve for the unknowns (usually
forces or angles).
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Vector Mechanics for Engineers: StaticsE i gh t h
E d i t i on
2 - 5
How ?
1. Imagine the particle to be isolated or cut free from itssurroundings.
2. Show all the forces that act on the particle.
Active forces: They want to move the particle.
Reactive forces: They tend to resist the motion.
3. Identify each force and show all known magnitudes
and directions. Show all unknown magnitudes and /
or directions as variables .
FBD at A Note : Engine mass = 250 Kg
A
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Vector Mechanics for Engineers: StaticsE i gh t h
E d i t i on
2 - 6
Sample Problem 2.4
In a ship-unloading operation, a
3500-lb automobile is supported bya cable. A rope is tied to the cable
and pulled to center the automobile
over its intended position. What is
the tension in the rope?
SOLUTION:
• Construct a free-body diagram for the
particle at the junction of the rope and
cable.
• Apply the conditions for equilibrium by
creating a closed polygon from the
forces applied to the particle.• Apply trigonometric relations to
determine the unknown force
magnitudes.
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Vector Mechanics for Engineers: StaticsE i gh t h
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Sample Problem 2.4
SOLUTION:
• Construct a free-body diagram for the
particle at A.
• Apply the conditions for equilibrium.
• Solve for the unknown force magnitudes.
°=
°=
° 58sinlb3500
2sin120sin AC AB T T
lb3570= ABT
lb144= AC T
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Vector Mechanics for Engineers: StaticsE i gh t h
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Sample Problem 2.6
It is desired to determine the drag force
at a given speed on a prototype sailboat
hull. A model is placed in a test
channel and three cables are used to
align its bow on the channel centerline.
For a given speed, the tension is 40 lb
in cable AB and 60 lb in cable AE .
Determine the drag force exerted on
the hull and the tension in cable AC .
SOLUTION:
• Choosing the hull as the free body,
draw a free-body diagram.
• Express the condition for equilibrium
for the hull by writing that the sum of
all forces must be zero.
• Resolve the vector equilibriumequation into two component
equations. Solve for the two unknown
cable tensions.
E E
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Vector Mechanics for Engineers: StaticsEi gh t h
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Sample Problem 2.6
SOLUTION:
• Choosing the hull as the free body, draw a
free-body diagram.
°=
==
25.60
75.1ft4
ft7tan
α
α
°=
==
56.20
375.0ft4
ft1.5tan
β
β
• Express the condition for equilibrium
for the hull by writing that the sum of
all forces must be zero.
0=+++= D AE AC AB F T T T Rrrrrr
E E
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Vector Mechanics for Engineers: StaticsEi gh t h
E d i t i on
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Sample Problem 2.6
• Resolve the vector equilibrium equation into
two component equations. Solve for the two
unknown cable tensions.
( ) ( )( ) ( )
( )
( )
( ) jT
iF T
R
iF F
iT jT iT
jT iT T
ji jiT
AC
D AC
D D
AC AC
AC AC AC
AB
r
r
r
rr
rr
rr
rrr
rr
rrr
609363.084.19
3512.073.34
0
lb069363.03512.0
56.20cos56.20sin
lb84.19lb73.3426.60coslb4026.60sinlb40
−++
++−=
=
=
−=+=
°+°=
+−=°+°−=
V M h i f E i S iE E
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Vector Mechanics for Engineers: StaticsEi gh t h
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Sample Problem 2.6
( )
( ) jT
iF T
R
AC
D AC r
r
r
609363.084.19
3512.073.34
0
−++
++−=
=
This equation is satisfied only if each component
of the resultant is equal to zero
( )( ) 609363.084.1900
3512.073.3400−+==
++−==∑∑
AC y
D AC x
T F F T F
lb66.19
lb9.42
+=
+=
D AC
F
T
V t M h i f E i St tiE E
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EXAMPLE
Write the scalar EofE:
+ → Σ Fx = TB cos 30º – TD = 0
+↑ Σ
Fy = TB sin 30º – 2.452 kN = 0
Solving the second equation gives: TB = 4.90 kN
From the first equation, we get: TD = 4.25 kN
Note : Engine mass = 250 Kg FBD at A
V t M h i f E i St tiE E
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SPRINGS, CABLES, AND PULLEYS
Spring Force = spring constant *
deformation, or
F = k * S
With a
frictionless
pulley, T1
= T2.
V t M h i f E i St tiE
E
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Vector Mechanics for Engineers: Staticsi gh t h
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EXAMPLE
Given: Sack A weighs 20 lb.
and geometry is as
shown.
Find: Forces in the cables and
weight of sack B.
Plan:
1. Draw a FBD for Point E.
2. Apply EofE at Point E to solvefor the unknowns (TEG & TEC).
3. Repeat this process at C.
V t M h i f E i St tiE i
E d
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EXAMPLE (continued)
The scalar EofE are:
+ → Σ Fx = TEG sin 30º – TEC cos 45º = 0
+ ↑ Σ Fy = TEG cos 30º – TEC sin 45º – 20 lbs = 0
Solving these two simultaneous equations for the
two unknowns yields:
TEC = 38.6 lb
TEG = 54.6 lb
A FBD at E should look like the one
to the left. Note the assumed
directions for the two cable tensions.
Vector Mechanics for Engineers: StaticsE i
E d
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EXAMPLE (continued)
+ → Σ Fx = 38.64 cos 45° – (4/5) TCD = 0
+ ↑ Σ Fy
= (3/5) TCD
+ 38.64 sin 45° – WB
= 0
Solving the first equation and then the second yields
TCD
= 34.2 lb and WB
= 47.8 lb .
The scalar EofE are:
Now move on to ring C.
A FBD for C should look
like the one to the left.