PHY 113 C Fall 2013 Lecture 8 1 PHY 113 C General Physics I 11 AM1215 P M MWF Olin 101 Plan for Lecture 8 Chapter 8 Conservation of energy Potential and kinetic energy for conservative forces ID: 331697
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Slide1
9/19/2013
PHY 113 C Fall 2013-- Lecture 8
1
PHY 113 C General Physics I
11 AM-12:15
P
M MWF Olin 101
Plan for Lecture 8:
Chapter 8 -- Conservation of energy
Potential and kinetic energy for conservative forces
Energy and non-conservative forces
PowerSlide2
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PHY 113 C Fall 2013-- Lecture 8
2Slide3
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Comments on preparation for next Thursday’s examSlide4
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Comments on preparation for next Thursday’s exam -- continuedSlide5
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Comments on preparation for next Thursday’s exam – continued
What you should bring to the exam (in addition to your well-rested brain):
A pencil or pen
Your calculator
An 8.5”x11” sheet of paper with your favorite equations
(to be turned
in
together with
the
exam)
What you should
NOT use during the
exam
Electronic devices (cell phone, laptop, etc.)
Your
textbookSlide6
Advice:
Keep basic concepts and equations at the top of your head.
Practice problem solving and math skills
Develop an equation sheet that you can consult.
Equation Sheet
Problem solving skills
Math skills
8/27/2013
PHY 113 A Fall 2013 -- Lecture 1
6Slide7
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PHY 113 C Fall 2013-- Lecture 8
7
iclicker
question
Which of the following best describes your opinion about the equation sheet:
I have not prepared my equation sheet
yet but hope to do so soon
I have a preliminary equation sheet but have not used it for doing homework
I have a preliminary equation sheet and have used it for doing homework
I am dreaming about preparing the equation sheet, but have not done it yet
In my opinion, the equation sheet is not really necessarySlide8
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Review of energy concepts:Slide9
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Some questions from
webassign
Assignment #7
In 1990 Walter
Arfeuille of Belgium lifted a 281.5-kg object through a distance of 17.1 cm using only his teeth.
(
a) How much work was done on the object
by
Arfeuille
in this lift, assuming the object was lifted at constant velocity?
T
mg
T
mg
y
f
y
iSlide10
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10
Some questions from
webassign
Assignment #7
In 1990 Walter
Arfeuille of Belgium lifted a 281.5-kg object through a distance of 17.1 cm using only his teeth.
How
much work was done on the object
by
gravity
?
T
mg
T
mg
y
f
y
iSlide11
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11
Some questions from
webassign
Assignment #7
What is the work done as the particle moves from
x=0
to
x=3
m?Slide12
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Some questions from
webassign
Assignment #7 -- continued
What is the work done as the particle moves from
x=8
to
x=10
m?Slide13
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Some questions from
webassign
Assignment #7
A 4.34-kg particle is subject to a net force that varies with position as shown in the figure. The particle starts from rest at
x
= 0.
What
is its speed at
x
= 5.00
m?Slide14
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Work and potential energy
Note: It is assumed that F is conservativeSlide15
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Work and potential energy -- continued
mg
mg
y
y
ref
Example – gravity near the surface of the Earth:Slide16
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Work and potential energy for gravity (near Earth’s surface)
mg
mg
y
f
y
i
y
refSlide17
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Work and potential energy continued
iclicker
question
Why is there a strange “-” sign in the definition of potential energy?
Physicists like to be annoying
No reason at all
There is a somewhat good reasonSlide18
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Work and potential energy continued
For the case that the total force acting on the system is conservative, we can use the definition of potential energy with the work-kinetic energy theoremSlide19
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Summary of work, potential energy, kinetic energy relationshipsSlide20
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Example
A block, initially at rest at a height h, slides down a
frictionless
incline. What is its final velocity?
h
h=0.5m
i
fSlide21
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Energy diagram
E
y
i
y
f
K
U
U, K, E (J)
mSlide22
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kSlide23
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Energy diagramsSlide24
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Example: Model potential energy function U(x) representing the attraction of two atomsSlide25
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Comment on relationship between potential energy and (conservative) force:Slide26
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Example: Mass sliding on frictionless looping track
i
iclicker
exercise:
In order for the ball completes the loop at A, what must the value of h?
h=R
h=2R
h>2R
Not enough information.Slide27
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i
Example: Mass sliding on frictionless looping track
0
0Slide28
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Another example; first
without
friction
Mass m
1
(=0.2kg) slides horizontally on a frictionless table and is initially at rest. What is its velocity when it moves a distance
D
x
=0.1m (and m
2
(=0.3kg) falls
D
y
=0.1m)?
iclicker
exercise:
What is the relationship of the final velocity of m
1
and m
2
?
They are equal
m
2
is faster than m
1
.
m
1
is faster than m2.Slide29
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Another example; first
without
friction
Mass m
1
(=0.2kg) slides horizontally on a frictionless table and is initially at rest. What is its velocity when it moves a distance
D
x
=0.1m (and m
2
(=0.3kg) falls
D
y
=0.1m)?
T
T
m
2
gSlide30
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Another example; now
with
friction
Mass m
1
(=0.2kg) slides horizontally on a table with kinetic friction and is initially at rest. What is its velocity when it moves a distance
D
x
=0.1m (and m
2
(=0.3kg) falls
D
y
=0.1m)?
T
T
m
2
g
fSlide31
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iclicker
exercise:
Assume a mass m starts at rest at A and moves on the
frictionless
surface as shown. At what position is the speed the largest?
A
B
C
none of these.Slide33
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PowerSlide34
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