Transcript Slide 1

PHYS-1600/2000 III4 Dissipation of Energy
Is the tension bigger
than, equal to, or smaller
than the weight of B?
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
What does the net force
look like? Does it depend
upon the direction of
motion?
1 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
AGENDA
ITEM
Introductory Concept Survey (Individual)
The Work-Energy Theorem and Non-Conservative Work
Is W-E Thm Always Valid? Analysis of Pushed Block w/Friction
Bouncing Ball Analysis
Fluid Drag and Terminal Velocity Motion
Survey Re-vote (Group Discussion Mode)
Dismissal
DURATION
START
0:10
0:15
0:25
0:25
0:20
0:10
0:00
0:10
0:25
0:50
1:15
1:35
1:45
ANNOUNCEMENTS
• Consider the concept questions about Newton’s 2nd Law.
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
2 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
1 of 3
A balloon is dropped a distance of 100 m.
Atmospheric drag is significant, causing
the balloon to fall at constant speed.
Through its flight the balloon lost 1 J of
gravitational potential energy (DPEg = -1
J). The amount of kinetic energy gained
(DKE) by the balloon is:
y
100
v = const
1. 1 J.
2. Zero.
3. -1 J.
0
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
3 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
2 of 3
A rocket (such as this SLS 130-ton-to-orbit
cargo lifter) blasts off from ground level.
During the first 10 seconds of its launch, it
accelerates uniformly upward and gains 1 GJ
of PEg. During that same time, the vehicle:
y
1. Loses 1 GJ of KE.
2. Neither loses nor gains KE.
3. Gains KE.
a
0
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
4 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
3 of 3
A ball drops from rest from 1 m of altitude, bounces, then reaches a maximum
final altitude of 0.5 m. It had 10 J of PEg initially, but only 5 J of PEg finally. The
work done on the ball by the normal force of the rigid surface was
1.
2.
3.
4.
DEAN SIEGLAFF
–5 J.
More than –5 J but less than zero.
Zero.
5 J.
NATHANIEL CUNNINGHAM
5 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
AGENDA
ITEM
Introductory Concept Survey (Individual)
The Work-Energy Theorem and Non-Conservative Work
Is W-E Thm Always Valid? Analysis of Pushed Block w/Friction
Bouncing Ball Analysis
Fluid Drag and Terminal Velocity Motion
Survey Re-vote (Group Discussion Mode)
Dismissal
DURATION
START
0:10
0:15
0:25
0:25
0:20
0:10
0:00
0:10
0:25
0:50
1:15
1:35
1:45
ANNOUNCEMENTS
• Consider the concept questions about Newton’s 2nd Law.
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
6 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
Non-conservative Forces and the General Work – Energy Theorem
The work done by a non-conservative (NC) force depends upon the path
of motion. Therefore, when NC forces are involved, the total mechanical
energy cannot be conserved. Rather, it can be dissipated or created.
ROCKET THRUST
AIR DRAG
Fthrust
Fdrag
DKE  0
DPE  0
mg
a
DKE  0
DPE  0
mg
DKE  DPE  WNC
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
7 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
Failure of the Work – Energy Theorem
There are cases whereby the mechanical energy of a system
cannot be fully accounted for (even taking into consideration
the work of NC forces).
•
•
Two blobs of clay of equal mass and opposite velocity colliding and
sticking together.
Two freight cars of equal mass and opposite velocity colliding and
coupling.
In these cases the WNC is ZERO. From the particle
dynamics view, a direct transformation from mechanical
energy E to thermal energy U has occurred. To find the
dissipative forces responsible, one must peer into the
internal structure of the bodies.
DKE  DPE  DU  0
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
8 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
Must be careful when applying work-energy principles to nonparticulate or non-rigid bodies:
•
•
•
•
•
Both particles and rigid bodies are structureless, ideal entities, not
thought of as being composed of any smaller interacting structures.
A particle or rigid body has no “internal degrees of freedom” in which
to store internal energy (thermal energy).
Real bodies such as blocks, crates, automobiles, and people, are
obviously not particles or rigid bodies.
The application of particle and rigid body mechanics to nonparticulate and non-rigid bodies will result in energetic inaccuracies.
Augmenting the principles of work and energy to include systems with
internal structure is called Thermodynamics.
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
9 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
The work of friction is not
computable by particulate means
(FDx), because its displacement is
not known.
The friction force arises from
interactions between bits and pieces
of the body and its environment that
are in motion relative to the body.
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
10 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
III4 Exit Homework Problem #1
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
11 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
III4 Exit Homework Problem #2
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
12 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
1 of 3
A balloon is dropped a distance of 100 m.
Atmospheric drag is significant, causing
the balloon to fall at constant speed.
Through its flight the balloon lost 1 J of
gravitational potential energy (DPEg = -1
J). The amount of kinetic energy gained
(DKE) by the balloon is:
y
100
v = const
1. 1 J.
2. Zero.
3. -1 J.
0
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
13 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
2 of 3
A rocket (such as this SLS 130 ton cargo
lifter) blasts off from ground level. During the
first 10 seconds of its launch, it accelerates
uniformly upward and gains 1 GJ of PEg.
During that same time, the vehicle:
y
1. Loses 1 GJ of KE.
2. Neither loses nor gains KE.
3. Gains KE.
a
0
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
14 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
3 of 3
A ball drops from rest from 1 m of altitude, bounces, then reaches a maximum
final altitude of 0.5 m. It had 10 J of PEg initially, but only 5 J of PEg finally. The
work done on the ball by the normal force of the rigid surface was
1.
2.
3.
4.
DEAN SIEGLAFF
–5 J.
More than –5 J but less than zero.
Zero.
5 J.
NATHANIEL CUNNINGHAM
15 of 15
PHYS-1600/2000 III4 Dissipation of Energy
NEBRASKA WESLEYAN UNIVERSITY
FALL 2014-2015
PROJECTION SCREEN
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III4: MARKED SEATS PLEASE HAND IN TODAY’S ACTIVITIES SHEETS
DEAN SIEGLAFF
NATHANIEL CUNNINGHAM
16 of 15