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VECTOR MECHANICS FOR ENGINEERS:
DYNAMICS
Eighth Edition
Ferdinand P. Beer
E. Russell Johnston, Jr.
Lecture Notes:
J. Walt Oler
Texas Tech University
CHAPTER
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13Kinetics of Particles: Energy Methods
For course 425203 Dynamics
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Introduction
• Previously, problems dealing with the motion of particles were solved through the fundamental equation of motion,Current chapter introduces two additional methods of analysis.
.amFrr
=
• Method of work and energy: directly relates force, mass, velocity and displacement.
• Method of impulse and momentum: directly relates force, mass, velocity, and time.
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Work of a Force
• Differential vector is the particle displacement.rdr
• Work of the force is
dzFdyFdxF
dsF
rdFdU
zyx ++=
=
•=
αcos
rr
• Work is a scalar quantity, i.e., it has magnitude and sign but not direction.
force. length ו Dimensions of work are Units are( ) ( )( )m 1N 1 J 1 =joule
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Work of a Force
• Work of a constant force in rectilinear motion,
( ) xFU ∆=→ αcos21
• Work of the weight is equal to product of weight W and vertical displacement ∆y.
• Work of the weight is positive when ∆y < 0, i.e., when the weight moves down.
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Work of a Force• Magnitude of the force exerted by a spring is
proportional to deflection,
( )lb/in.or N/mconstant spring =
=
k
kxF
• Work of the force exerted by spring is positive when x2 < x1, i.e., when the spring is returning to its undeformed position.
• Work of the force exerted by the spring is equal to negative of area under curve of F plotted against x,
( ) xFFU ∆+−=→ 2121
21
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Work of a Force
Forces which do not do work (ds = 0 or cos α = 0):
• weight of a body when its center of gravity moves horizontally.
• reaction at a roller moving along its track, and
• reaction at frictionless surface when body in contact moves along surface,
• reaction at frictionless pin supporting rotating body,
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Particle Kinetic Energy: Principle of Work & Energy
dt
dvmmaF tt ==
• Consider a particle of mass m acted upon by force Fr
• The work of the force is equal to the change in kinetic energy of the particle.
Fr
• Units of work and kinetic energy are the same:
JmNms
mkg
sm
kg2
22
21 =⋅=
=
== mvT
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Power and Efficiency• rate at which work is done.
vF
dt
rdF
dt
dU
Power
rr
rr
•=
•==
=
• Dimensions of power are work/time or force*velocity. Units for power are
W746slbft
550 hp 1orsm
N 1sJ
1 (watt) W 1 =⋅
=⋅==
•
inputpower outputpower
input workkoutput wor
efficiency
=
=
=η
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Sample Problem 13.1An automobile weighing 4000 N is driven down a 5o incline at a speed of 88 m /s when the brakes are applied causing a constant total breaking force of 1500 N.
Determine the distance traveled by the automobile as it comes to a stop.
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Sample Problem 13.1
m 6.1345=x
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Sample Problem 13.2
Two blocks are joined by an inextensible cable as shown. If the system is released from rest, determine the velocity of block A after it has moved 2 m. Assume that the coefficient of friction between block Aand the plane is µk = 0.25 and that the pulley is weightless and frictionless.
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Sample Problem 13.2
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Sample Problem 13.3
A spring is used to stop a 60 kg package which is sliding on a horizontal surface. The spring has a constant k = 20 kN/m and is held by cables so that it is initially compressed 120 mm. The package has a velocity of 2.5 m/s in the position shown and the maximum deflection of the spring is 40 mm.
Determine (a) the coefficient of kinetic friction between the package and surface and (b) the velocity of the package as it passes again through the position shown.
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Sample Problem 13.3
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Sample Problem 13.3
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Sample Problem 13.4
A 2000 N car starts from rest at point 1 and moves without friction down the track shown.
Determine:
a) the force exerted by the track on the car at point 2, and
b) the minimum safe value of the radius of curvature at point 3.
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Sample Problem 13.4
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Sample Problem 13.4
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Potential Energy
2121 yWyWU −=→
• Work of the force of gravity ,Wr
• Units of work and potential energy are the same:
JmN =⋅== WyVg
• Choice of datum from which the elevation y is measured is arbitrary.
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Potential Energy• Work of the force exerted by a spring depends
only on the initial and final deflections of the spring,
222
1212
121 kxkxU −=→
• The potential energy of the body with respect to the elastic force,
( ) ( )2121
221
ee
e
VVU
kxV
−=
=
→
• Note that the preceding expression for Ve is valid only if the deflection of the spring is measured from its undeformed position.
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Conservative Forces• Concept of potential energy can be applied if the
work of the force is independent of the path followed by its point of application.
( ) ( )22211121 ,,,, zyxVzyxVU −=→
Such forces are described as conservative forces.
• For any conservative force applied on a closed path,
0=•∫ rdFrr
• Elementary work corresponding to displacement between two neighboring points,
( ) ( )( )zyxdV
dzzdyydxxVzyxVdU
,,
,,,,
−=
+++−=
Vz
V
y
V
x
VF
dzz
Vdy
y
Vdx
x
VdzFdyFdxF zyx
grad−=
∂∂
+∂∂
+∂∂
−=
∂∂
+∂∂
+∂∂
−=++
r
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Sample Problem 13.6
A 20 N collar slides without friction along a vertical rod as shown. The spring attached to the collar has an undeflected length of 4 cm and a constant of 3 N/cm.
If the collar is released from rest at position 1, determine its velocity after it has moved 6 cm. to position 2.
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Sample Problem 13.6
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Example
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Example
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Example
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Homework
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Homework
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Homework