9/24/2012phy 113 a fall 2012 -- lecture 111 phy 113 a general physics i 9-9:50 am mwf olin 101...
DESCRIPTION
9/24/2012PHY 113 A Fall Lecture 113 Comments about Exam 1 Scores 70 ≤ G ≤ 100 Please keep working hard, even if your score is 90 ≤ G ≤ 100 Please make an appointment to see me if your score is 70 ≤ G ≤ 90 Solutions will be posted on the web on the course website (you will have to login with your WFU login and password)TRANSCRIPT
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PHY 113 A Fall 2012 -- Lecture 11 19/24/2012
PHY 113 A General Physics I9-9:50 AM MWF Olin 101
Plan for Lecture 11:
Chapter 7 -- The notion of work
1. Definition of work
2. Examples of work
3. Potential energy and work; conservative forces
4. Comments about Exam 1
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PHY 113 A Fall 2012 -- Lecture 11 29/24/2012
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PHY 113 A Fall 2012 -- Lecture 11 39/24/2012
Comments about Exam 1
Scores 70 ≤ G ≤ 100 Please keep working hard, even if your
score is 90 ≤ G ≤ 100 Please make an appointment to see me
if your score is 70 ≤ G ≤ 90 Solutions will be posted on the web on
the course website (you will have to login with your WFU login and password)
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PHY 113 A Fall 2012 -- Lecture 11 49/24/2012
Force effects acceleration
A related quantity is Work rFr
rdW
f
i
fi
F
dr
ri rj
Energy work, kinetic energy
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PHY 113 A Fall 2012 -- Lecture 11 59/24/2012
Definition of vector “dot” product
q
0 then ,90 if that Note
cos
BABA
o
AB
q
q
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PHY 113 A Fall 2012 -- Lecture 11 69/24/2012
Definition of vector “dot” product
q
(scalar) 5.37120cos)15)(5(
120 ,15 ,5 :Example
cos
o
o
BA
BAq
q
BA
AB
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PHY 113 A Fall 2012 -- Lecture 11 79/24/2012
Definition of vector “dot” product -- continued
q
yyxx
yxyx
BABA
BBAA
BA
jiBjiA ˆˆ and ˆˆ Suppose
Note that the result of a vector dot product is a scalar.
103412 ˆ31̂ and ˆ4ˆ2 :Example
BAjiBjiA
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PHY 113 A Fall 2012 -- Lecture 11 89/24/2012
rFr
rdW
f
i
fi
Definition of work: F
dr
ri rj
cal 0.239 J 1
JoulesmetersNewtons Work : workof Units
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PHY 113 A Fall 2012 -- Lecture 11 99/24/2012
Units of work:
work = force · displacement = (N · m) = (joule)
• Only the component of force in the direction of the
displacement contributes to work.• Work is a scalar quantity.• If the force is not constant, the integral form must be used.• Work can be defined for a specific force or for a combination of
forces
rFr
rdW
f
i
11 rFr
rdW
f
i
22 212121 )( WWdWf
i
rFFr
r
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PHY 113 A Fall 2012 -- Lecture 11 109/24/2012
iclicker question: A ball with a weight of 5 N follows the trajectory shown. What is the work done by gravity from the initial ri to final displacement rf?
(A) 0 J (B) 7.5 J (C) 12.5 J (D) 50 J
1m1m
2.5m
ri
rf
10 m
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PHY 113 A Fall 2012 -- Lecture 11 119/24/2012
mg
ri
rf
W=mg(rfri)<0
mg
ri
rf
W=mg(rfri)>0
Gravity does negative work:
Gravity does positive work:
0r 0r
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PHY 113 A Fall 2012 -- Lecture 11 129/24/2012
Work done by a variable force: rFr
rdW
f
i
fi
f
i
f
i
x
xxfi dxFdW rF
r
r
:case In this
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PHY 113 A Fall 2012 -- Lecture 11 139/24/2012
Example:
JmNmNdxFdWf
i
f
i
x
xxfi 2525)4)(5( 2
1 rFr
r
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PHY 113 A Fall 2012 -- Lecture 11 149/24/2012
Example – spring force: Fx = - kx
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PHY 113 A Fall 2012 -- Lecture 11 159/24/2012
x
F
Positive work
Negative work
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PHY 113 A Fall 2012 -- Lecture 11 169/24/2012
Detail:
22212
21
if
x
x
x
x
x
xxfi xxkkxdxkxdxFdW f
i
f
i
f
i
f
i
rFr
r
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PHY 113 A Fall 2012 -- Lecture 11 179/24/2012
More examples:
Suppose a rope lifts a weight of 1000N by 0.5m at a constant upward velocity of 4.9m/s. How much work is done by the rope?
(A) 500 J (B) 750 J (C) 4900 J (D) None of these
Suppose a rope lifts a weight of 1000N by 0.5m at a constant upward acceleration of 4.9m/s2. How much work is done by the rope?
(A) 500 J (B) 750 J (C) 4900 J (D) None of these
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PHY 113 A Fall 2012 -- Lecture 11 189/24/2012
iclicker exercise:Why should we define work?
A. Because professor like to torture students.
B. Because it is always good to do workC. Because it will help us understand
motion. D. Because it will help us solve the energy
crisis.
Next time we will discuss the Work-Kinetic energy theorem.
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PHY 113 A Fall 2012 -- Lecture 11 199/24/2012
FPq
mg
n
fk
xi xf
Assume FP sinq <<mg
Work of gravity? 0
Work of FP? FP cos q (xf-xi)
Work of fk?mkn (xf-xi)=mk(mg- FP sin q (xf-xi)
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PHY 113 A Fall 2012 -- Lecture 11 209/24/2012
A man must lift a refrigerator of weight mg to a height h to get it to the truck.
For which method does the man do more work:A. Vertically lifting the refrigerator at constant
speed to height h?B. Moving the refrigerator up the ramp of length
L at constant speed with h=L sin q.
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PHY 113 A Fall 2012 -- Lecture 11 219/24/2012
iclicker exercise:Which of the following statements about friction forces are true.
A. Friction forces always do positive work.B. Friction forces always do negative work.C. Friction forces can do either positive or
negative work.