mechanics of solids by crandall,dahl,lardner, 1st chapter
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Mechanics of solids by crandall,dahl,lardner, 1st chapterTRANSCRIPT
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Mechanics of Solids Vikas Cahudhari 1
Chapter: 1
Fundamental Principles of Mechanics
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Mechanics of Solids Vikas Cahudhari 2
Mechanics
Study of Force & Motion
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Mechanics of Solids Vikas Cahudhari 3
MechanicsStudy of Force & Motion
Gross/Overall Motion
(Dynamics)
KINETICS KINEMATICS
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Mechanics of Solids Vikas Cahudhari 4
MechanicsMotion
Localized Motion (Deformation)
(STATICS)
Rigid Bodies Deformable bodies
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Topics to be addressed for Ch-1
• Force• Moment• Types of loading• Types of Support & related Reactions• Free Body Diagrams• Equilibrium Conditions• Trusses & Frames• Friction
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DefinitionsMatter: Matter is actually made up of atoms and molecules. Since this is too complex, matter is taken to be continuously distributed-a continuum; it may be rigid or deformable.
Particle: A material with negligibly small dimensions is called a 'particle', the mass being concentrated at a point. The concept is analogous to that of a point in geometry, which has position but no dimensions.
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However, there is a difference in that a particle, being a material, has mass while a point, being only a geometrical concept, has none.
Body: A collection of particles is called a 'body'. It may be a rigid body or an elastic or deformable body.
Rigid Body: The particles in a rigid body are so firmly connected together that their relative positions do not change irrespective of the forces acting on it. Thus the size and shape of a rigid body are always maintained constant
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Mechanics of Solids Vikas Cahudhari 8
Elastic Body: A body whose size and shape can change under forces is a deformable body. When the size and shape can be regained on removal of forces, the body is called an elastic body.
Scalar Quantity: A quantity which is fully described by its magnitude only is a scalar. Arithmetical operations apply to scalars. Examples are: Time, mass, area and speed.
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Mechanics of Solids Vikas Cahudhari 9
Vector Quantity: A quantity which is described by its magnitude and also its direction is a vector. Operations of vector algebra are applicable to vectors. Examples are: Force, velocity, moment of a force and displacement .
Mass: The quantum of matter in a body is characterized by its 'mass', which is measure of its inertia, or its resistance to change of velocity. It is the property of a body by virtue of which it can experience attraction to other bodies. The symbol used is 'm'.
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Mechanics of Solids Vikas Cahudhari 10
Analysis of Engineering systemsStudy of forces • (Tensile, Compressive, Shear, Torque, Moments)
Study of motion• (Straight, curvilinear, displacement, velocity,
acceleration.)Study of deformation• (Elongation, compression, twisting)
Application of laws relating the forces to the deformation
In some special cases one or more above mentioned steps may become trivial
e.g. For rigid bodies deformation will be negligible.If system is at rest, position of system will be independent of time.
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Step-By-Step procedure to solve problems in mechanics of solids
Select actual/real system of interest.
Make assumptions regarding desired characteristics of the system
Develop idealized model of the system(Structural and Machine elements)
Apply principles of mechanics to the idealized model to compare these calculated results with the behaviour of the actual system
Contd…
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If the results (calculated and actual) differ widely repeat above steps till satisfactory idealized model is obtained.
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ForceForce: In physics, a net force acting on a body causes that body to accelerate; that is, to change its velocity. The concept appeared first in the second law of motion of classical mechanics.
There are three basic kinds of forces as mentioned below
•Tensile force or pull
•Compressive force or push
•Shear force
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Tensile Force or Pull
•When equal and opposite forces are applied at the ends of a rod or a bar away from the ends, along its axis, they tend to pull the rod or bar. This kind of a force is called a tensile force or tension.
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Compressive Force or Push
•When equal and opposite forces act at the ends of a rod or a bar towards the ends along its axis, they tend to push the rod or the bar. This kind of force is called a compressive force or compression.
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Shear Force
When equal and opposite forces act on the parallel faces of a body, shear occurs on these planes. This tends to cause an angular deformation as shown.
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• One Newton is force which gives an acceleration of 1 m/s2 to a mass of 1 kg.
• A mass of 1kg on earth’s surface will experience a gravitational force of 9.81 N.
• Hence a mass of 1kg has because of gravitational force of the earth, a weight of 9.81 N.
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Mechanics of Solids Vikas Cahudhari 18
System of Forces
Coplanar Non-CoplanarCollinear
Concurrent
Parallel, Non-concurrent
Non-concurrent & Non-Parallel
Concurrent
Parallel
Non-concurrent & Non-Parallel
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Mechanics of Solids Vikas Cahudhari 19
System of Force • Terms to be familiar with
– Concurrent Forces (a)
– Parallel Forces (b)
– Line of Action (c)
– Coplanar Forces
(c)
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Mechanics of Solids Vikas Cahudhari 20
Forces
Coplanar & Non-Concurrent
F
R
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Resultant Force
If a force system acting on a body can be replaced by a single force, with exactly the same effect on the body, this single force is said to be the 'resultant' of the force system.
RF3F2F1
=
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Law of parallelogram of forces:
If two forces acting at a point are represented in magnitude and direction by the adjacent sides of a parallelogram, then their resultant is represented by the diagonal passing through the point of intersection of the two sides representing the forces.
P
F1
F2
F F = (F12 + F2
2 + 2F1 F2 cosθ)1/2
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Lami’s Theorem
If three forces acting at a point are in equilibrium, each force is proportional to the sine of the angle between the other two.
O αβ
γP Q
Rγβα sinsinsin
RQP==
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Superposition PrincipalIf a system is acted up on a set of forces, then the net effect of these forces is equal to the summation net effect of individual force.
This principle is applicable for linear systems.
F1 F3
F2
F1
F2
F3
= + +
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The moment of force
The moment of force F about a point O is r X F.
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Moment = r×FThe moment itself is a vector quantity.Its direction is perpendicular to the
plane determined by OP and F. The sense is fixed by the right hand ruleFrom calculus the magnitude of the
cross product r×F = F r sinφ, Where r sinφis the perpendicular distance between point O and vector F.
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F
x
Moment M = F X x
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Ex. 1 on Moment
4 m
15 kN
30o
O
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Solution for Ex. 1 on Moment
4 m
15 kN
30o
15 sin 30 kN
O
Mo = 15sin30 kN X 4 m = 30 kN-m (CCW)
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Ex. 2 on Moment
10 kN
4 m
O
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Solution for Ex. 2 on Moment
10 kN
4 m
O
Mo = 10 kN X 2 m = 20 kN-m (CW)
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Ex. 3 on Moment
10 kN
4 m
2 m O
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Solution for Ex. 3 on Moment
10 kN
4 m
2 m O
Mo = 10 kN X 2 m = 20 kN-m (CW)
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Ex. 4 on Moment
Mz = F kN X d m = Fd kN-m
x
y
zd
F
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Ex. 5 on Moment
x
y
zd
F
My = F kN X d m = Fd kN-m
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Ex. 6 on Moment
MR = (Mz2 + My
2)1/2 kN-m
x
y
zd
F2
F1
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Mechanics of Solids Vikas Cahudhari 38
CALCULATE THE MOMENT 400 N FORCE ABOUT THE POINT O
400 sin 60
400 cos 6040 mm
Let, CCW moments are +ve and CW -ve
MO = 400 cos 60 (40)x10-3 – 400 sin 60 (120)x10-3
MO = - 33.6 Nm MO = 33.6 Nm (CW)
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Sum of the moments of both the forces about B, we have total moment
MB = F * (a+d) - (F) * a = Fd (CCW)
d - distance between the forces.
d a
F F
BA
A special case of moments is a couple. A couple consists of two parallel forces that are equal in magnitude, opposite in direction. It does not produce any translation, only rotation. The resultant force of a couple is zero. BUT, the resultant of a couple is not zero; it is a pure moment.
Couple
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About couple….
• This result is independent of the location of B.
• Moment of a couple is the same about all points in space.
• A couple may be characterized by a moment vector without specification of the moment center B, with magnitude Fd.
• Encircling arrow indicates moment of a couple.
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Mechanics of Solids Vikas Cahudhari 41
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Types of Loading
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Types of Loading1. Concentrated Load / Point Load
F F
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Types of Loading2. Uniformly Distributed Load (UDL)
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Types of Loading3. Uniformly Varying Load (UVL)
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Types of Loading4. Uniformly Varying Load (UVL)
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Types of Loading5. Moment (Pure Bending Moment)
MB
x
y
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Types of Loading6. Moment
MT
x
y
z
y
z
MT Twisting Moment
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Support &
Reaction at the Support
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Types of Supports & Reactions
1. Simple SupportF
F
RA RB
A B
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Types of Supports & Reactions
2. Simple Support with hinge at A & B
F
RA RB
F
A B
HAHB
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Types of Supports & Reactions
3. Roller SupportF
F
RA RB
A B
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Types of Supports & Reactions4. Roller support & Simple support
F
F
RA RB
A B
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Types of Supports & Reactions5. Fixed support & roller support
F
A B
F
(RA)V (RB)V
(RA)H
MA
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Types of Supports & Reactions
6. Smooth/Frictionless Support
N
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Types of Supports & Reactions
7. Frictional Support
N
FS = μN
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Force Transmitting properties of some idealized mechanical elements
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Mechanics of Solids Vikas Cahudhari 60
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Mechanics of Solids Vikas Cahudhari 61
Equilibrium Conditions
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Mechanics of Solids Vikas Cahudhari 62
If the resultant force acting on a particle is zero then that can be called as equilibrium
Dynamic EquilibriumThe body is said to be in equilibrium condition when the acceleration is zero
Static EquilibriumThe static body is in equilibrium condition if the resultant force acting on it is zero
Equilibrium
Static Eqlbm Dynamic Eqlbm
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Mechanics of Solids Vikas Cahudhari 63
Summation of all the FORCES should be zero
Σ F = 0Summation of all the MOMENTS of all the forces about any arbitrary point should be zero
Σ M = 0
Necessary and sufficient condition
for body to be in Equilibrium
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Mechanics of Solids Vikas Cahudhari 64
Two Force Member
• Two forces can’t have random orientation, must be along AB & FA=FB
FAFB
Non-equilibrium condition Equilibrium Condition
FA
FB
A
B
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Mechanics of Solids Vikas Cahudhari 65
Three Force Member• Three forces can’t have random orientation
• All must lie in the plane ABC if total moment about each of the points A,B & C is to vanish.
• They must all intersect in a common point O.
FA
FB
FC
FA FB
FC
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Mechanics of Solids Vikas Cahudhari 66
GENERAL COPLANER FORCE SYSTEM
• External forces acting on a system in equilibrium all lie in the plane of sketch.
• Three of six general scalar equations of equilibrium are satisfied: no force components perpendicular to the plane , & If moments are taken about a point O lying in the plane, the only moment components will be perpendicular to the plane.
• Hence for 2-dimensional problem, three independent scalar conditions of equilibrium remains.
i.e ∑FX = 0∑FY =0 and∑M =0
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Mechanics of Solids Vikas Cahudhari 67
• If it is possible to determine all the forces involved by using only the equilibrium requirements without regard to deformations, such systems are called statically determinate.statically determinate.
• Use of equilibrium conditions to solve for unknown forces from known forces.
Statically Determinate System
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Mechanics of Solids Vikas Cahudhari 68
Statically Indeterminate Systems
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Mechanics of Solids Vikas Cahudhari 69
Ex. 1.8 CrandalStatically Indeterminate Situation
1 kN
3.0 m
2.2 m
BA
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Mechanics of Solids Vikas Cahudhari 70
Engineering Application
• STEPS IN GENERAL METHOD OF ANALYSIS1. SELECTION OF SYSTEM2. IDEALIZATION OF SYSTEM CHARACTERISTICS
• FOLLOWED BY1. STUDY OF FORCES & EQUILIBRIUM
REQUIREMENTS2.STUDY OF DEFORMATION & CONDITIONS OF
GEOMETRIC FIT3.APPLICATION OF FORCE-DEFORMATION
RELATIONS
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Mechanics of Solids Vikas Cahudhari 71
Free –body diagramThe sketch of the isolated component from a system / and all the external forces acting on it is often called a free- body diagram
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Mechanics of Solids Vikas Cahudhari 72
A
B
W
RAB
FBD for B
A
B
RZ
RX
RY
W
W
RY
X
Y
Z
W
RZ
RBA
FBD for A
RX
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Mechanics of Solids Vikas Cahudhari 73
A
B
30o
W
W
X
Y
Z
W
RBA
FBD for B
RX
W
N
RBA
FBD for A
RZ
RY
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Mechanics of Solids Vikas Cahudhari 74
Example
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Mechanics of Solids Vikas Cahudhari 75
Determine the forces at B and C
Idealization
1. Neglect the friction at pin joints
2. Ignore the self weight of rods
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Mechanics of Solids Vikas Cahudhari 76
Free body diagram of the frame
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Mechanics of Solids Vikas Cahudhari 77
Free body diagrams of bar BD and CD i.e. Isolation from the system
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Mechanics of Solids Vikas Cahudhari 78
Σ Fy= 0
FB sin 45- 20 = 0
FB = 28.28 kN
Σ Fx= 0
FC - FB cos 45 = 0
FC = 20 kN
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Mechanics of Solids Vikas Cahudhari 79
Trusses
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Mechanics of Solids Vikas Cahudhari 80
Trusses• Trusses are designed to support the loads
and are usually stationary, fully constrained structures. All the members of a truss are TWO FORCE members
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Mechanics of Solids Vikas Cahudhari 81
Use of truss
1. Construction of roof
2. Bridges
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Mechanics of Solids Vikas Cahudhari 82
3. Transmission line towers.
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Mechanics of Solids Vikas Cahudhari 83
Relation between number of joints and member in trusses
m = 2j - 3
If a truss obeys the equation, is called Perfect truss (just rigid truss)
If no. of members are more than this equation then it is called Redundant truss (over rigid truss)
If no. of members are less than this equation then it is called Deficient truss (under rigid truss)
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Mechanics of Solids Vikas Cahudhari 84
Method of analysis of truss
1.Joint equilibrium method
2.Section equilibrium method
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Mechanics of Solids Vikas Cahudhari 85
Joint Equilibrium MethodIn this method using FBD for each joint, equilibrium of the joints of the truss is consider one at a time. At each joint it is necessary to satisfy the static equilibrium equations. This method is suitable, when it is asked to calculate forces in all the members of the truss
Only two static equilibrium equations will be satisfied at a time because Σ M = 0 will always be satisfied for concurrent force system.
Only two unknown forces will be determined at a time.
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Mechanics of Solids Vikas Cahudhari 86
Step by step procedure of joint equilibrium method
1. Draw a FBD of an entire truss showing the external forces acting on it. (i.e. applied forces/ loads and reactions)
2. Obtain the magnitude and direction of reactions by using the static equilibrium equations.
3. Select a joint where only two unknown forces exit and draw its FBD assuming all unknown forces tensile.
4. Apply the equilibrium equations to the joint, which will give the forces in the members.
5. If the obtained value is –ve then our assumed direction is wrong. Change the nature of force.
6. Apply the above steps to each joint and determine the forces.
7. At the end, redraw the truss and show the correct direction as well as its magnitude of force in each member.
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Mechanics of Solids Vikas Cahudhari 87
W
F
AC
B
W
F
CYAY
AX
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Mechanics of Solids Vikas Cahudhari 88
A C
B
FAB
FAB FBC
FBC
FACFAC
Compression
Tension
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Mechanics of Solids Vikas Cahudhari 89
Calculate the force in each member of the loaded truss.
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Mechanics of Solids Vikas Cahudhari 90
Σ FX = 0 = CX – 2 CX = 2 KN
Σ MC = 0 = 2 x 3 – AY x 6 AY = 1 KN
2 KN
CX
CYAY
Σ FY = 0 = AY - CY CY = 1 KN
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Mechanics of Solids Vikas Cahudhari 91
A
AY
FAB
FAE
At Joint A
Σ FY = 0 = AY - FAE sin 45 --- FAE = 1.414 kN (C)
Σ FX = 0 = FAB – FAE cos 45 ---- FAB = 1 kN (T)
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Mechanics of Solids Vikas Cahudhari 92
A
AY
FAB
FAE
At Joint E
Σ FY = 0 = FAE cos 45 - FBE --- FBE = 1 kN (T)
Σ FX = 0 = FAE sin 45 - FDE ---- FDE = 1 kN (C)
FBE
FDEE
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Mechanics of Solids Vikas Cahudhari 93
A
AY
FAB
FAE
At Joint B
Σ FY = 0 = FBE - FBD sin 45 --- FBD = 1.414 kN (C)
Σ FX = 0 = FBC – FAB – FBD cos 45 -- FBC = 2 kN (T)
FBC
FBE
FDEE
B
FBD
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Mechanics of Solids Vikas Cahudhari 94
A
AY
FAB
FAE
At Joint C
Σ FY = 0 = FCD - CY --- FCD = 1 kN (T)
FBC
FBE
FDE
FCD
2 kN
CY
CX
E
B
FBD
C
D
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Mechanics of Solids Vikas Cahudhari 95
Compressive1 kNDE7Tensile1 kNCD6Compressive1.414 kNBD5Tensile2 kNBC4Tensile1 kNBE3Tensile1 kNAB2Compressive1.414 kNAE1Nature of forceForceMemberSr. No.
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Mechanics of Solids Vikas Cahudhari 96
Section equilibrium methodThis method is also based upon the conditions of static equilibrium. This method is suitable when forces only in few of the members of the truss, particularly away from the supports are required.
Conditions of equilibrium are
Σ FX = 0 Σ FY = 0 Σ M = 0
Where Σ M is the moments of all the forces about any point in the plane of the forces.
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Mechanics of Solids Vikas Cahudhari 97
Since we have only three equilibrium equation, only maximum three unknown forces can be found at a time. Therefore a care should be taken while taking a section that section line does not cut more than three members in which the forces are unknown.
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Mechanics of Solids Vikas Cahudhari 98
Step by step procedure of section equilibrium method
1. Draw a FBD of an entire truss showing the external forces acting on it. (i.e. applied forces/ loads and reactions)
2. Obtain the magnitude and direction of reactions by using the static equilibrium equations.
3. Draw an imaginary line which cuts the truss and passes through the members in which we need to find the forces, care must be taken to see that the section line must cut maximum of three members in which forces are unknown.
4. Prepare FBD of either part showing loads, reactions and internalforces in the members, which are cut by section line.
5. Apply the three conditions of equilibrium. If more than three unknowns are to be calculated, then two section lines can be selected and then the truss is to be solved by the above procedure.
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Mechanics of Solids Vikas Cahudhari 99
Determine the forces in members BC, BF, FJ
100 kN 100 kN
50 kN
A
J F E
DCB
9 m
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Mechanics of Solids Vikas Cahudhari 100
100 kN 100 kN
50 kN
A
J F E
DCB
AY
AX
DY
Σ MA = 0 = DY X 9 – 100 X 3 – 100 X 6 – 50 X 3 sin 60
DY = 114.43 kN
Σ FX = 0 = 50 – AX ------------ AX = 50 kN
Σ FY = 0 = AY – 100 – 100 + DY ------- AY = 85.57 kN
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Mechanics of Solids Vikas Cahudhari 101
100 kN 100 kN
50 kN
A
J F E
DCB
AY
AX
DY
FFJ
FBF
FBC
Taking moment about point B
Σ MB = 0 = - AY X 3 + FFJ X 3 sin 60 ----- FFJ = 98.80 kN (C)
Σ FY = 0 = AY + FBF sin 60 – 100 ------ FBF = 16.66 kN (T)
Σ FX = 0 = FBC + FBF cos 60 – FFJ – AX ------- FBC = 140. 47 kN (T)
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Mechanics of Solids Vikas Cahudhari 102
Compressive98.80 kNFJ3Tensile16.66 kNBF2Tensile140.47 kNBC1Nature of forceForceMemberSr. No.
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Mechanics of Solids Vikas Cahudhari 103
FrictionFriction forces are set up whenever a tangential force is applied to a body pressed normally against the surface of another
(a) Body A pressed against B
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Mechanics of Solids Vikas Cahudhari 104
FBD of A
FBD of B
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Mechanics of Solids Vikas Cahudhari 105
Friction forceis the result of interaction of the surface
layers of bodies in contact
Two separate friction forces are given byFS = fs N Where FS is static friction forceFK = fk N Where FK is kinetic friction force
fs & fk are static and kinetic coefficients of friction and are intrinsic properties of the interface between the materials in contact.
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Mechanics of Solids Vikas Cahudhari 106
Main Properties of Friction Force (F)
1. If there is no relative motion between a & b,
then the frictional force (F) is exactly equal &
opposite to the applied tangential force (T).
2. This condition can be maintained for any
magnitude of T between zero & certain limiting
value Fs, called the static friction force.static friction force.
3. IF T > FS, sliding will occur.
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Mechanics of Solids Vikas Cahudhari 107
4. If body A slides on body B, then F acting on
body A will have a directiondirection oppositeopposite to velocity
of A relative to B, & its magnitude will be Fk, called the kinetic friction force.kinetic friction force.
• FS and FK are proportional to normal force N.
»FS= fS N, FK =fk N
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Mechanics of Solids Vikas Cahudhari 108
Both coefficients,Depends on materials of bodies in contactDepends on state of lubrication/contamination
at the interfaceIndependent of the area of the interfaceIndependent of the roughness of the two
interfacesIndependent of time of contact and relative
velocity respectively
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Mechanics of Solids Vikas Cahudhari 109
It is of interest to find the range of values of W which will hold the block of weight WO = 500 N in equilibrium of the inclined plane if the coefficient of static friction is fS = 0.5.
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Mechanics of Solids Vikas Cahudhari 110
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Mechanics of Solids Vikas Cahudhari 111
Estimate the force in the link AB when a man of weight equal to 800 N stands on top of the ladder.
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Mechanics of Solids Vikas Cahudhari 112
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Mechanics of Solids Vikas Cahudhari 113
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Mechanics of Solids Vikas Cahudhari 114
Draw FBD of blocks A & B
A
(1000 N)
B
(2000 N)P
µ1
µ2
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Mechanics of Solids Vikas Cahudhari 115
Draw FBD of blocks A & B
A
(300 N)
B
(900 N)µ1
µ2 θ
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Mechanics of Solids Vikas Cahudhari 116
Draw FBD of blocks A & B
A
W kN B
(5 kN)
µ1
µ2
30o
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Mechanics of Solids Vikas Cahudhari 117
Problem:
A ladder of weight 390 N and 6 m long is placed against a vertical wall at an angle of 300 as shown in figure. The coefficient of friction between the ladder and the wall is 0.25 and that between ladder and floor is 0.38. Find how high a man of weight 1170 N can ascend, before the ladder begins to slip.
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Mechanics of Solids Vikas Cahudhari 118
Problem
A cantilever truss is loaded as shown in figure. Find the value of W which will produce a force of 150 kN magnitude in the member AB.
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Mechanics of Solids Vikas Cahudhari 119
Problem
By using the method of sections find the forces in the members BC, BF and BE of the cantilever truss shown in figure.
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Mechanics of Solids Vikas Cahudhari 120
What we studied in Ch-1• Force• Moment• Types of loading• Types of Support & related Reactions• Free Body Diagrams• Equilibrium Conditions• Trusses• Friction
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Mechanics of Solids Vikas Cahudhari 121
Numericals are based on
• Free Body Diagrams• Force/Reaction Determination• Trusses • Friction