resonance lecture 32 november 21, 2008. robert hooke “ceiiinosssttuv” anagram for “ut tensio,...
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Resonance
Lecture 32November 21, 2008
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Robert Hooke
• “ceiiinosssttuv”• Anagram for “ut tensio, sic vis”• “as the extension, so the force”
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Workbook Problems due Friday
• Problems 14-1 through 8, pages 14-1 -- 5
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Energy in Simple Harmonic Motion2
2 2
2
2
1
21 1
constant2 2
1E(at x= A)=U
21
(at x=0)=K2
1
2
S
MAX
MAX MAX
MAX
U kx
E K U mv kx
kA
E mv
kv A
m
k kf
m m
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Pendulum1
2
g gf
L L
Point mass on a string
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Physical Pendulum
θ
Center of gravityL
d
mgd
I
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Damped Harmonic Motion
Friction rears its ugly head!
( )t
MAXx t Ae
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Damped Harmonic Motion
0 2 4 6 8 10 12
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
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Problem 14.15
A) When the displacement of a mass on a spring is ½A, what fraction of the mechanical energy is kinetic energy and what fraction is potential?
2 22
2
1 1 1
2 2 2 2 4
1
21
43
4
S
S
A AU kx k k
E kA
U
EK
E
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B) At what displacement as a fraction of A, is the energy half kinetic and half potential?
2 22
2
2
1 1 1*
2 2 2
2 2
SU kx kA
A Ax
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Problem 14.30The center of gravity of a lower leg of a cadaver which had a mass of 5kg was located 18cm from the knee. When pivoted at the knee , the oscillation frequency was 1.6Hz. What is the moment of inertia of the lower leg?
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Problem 14.30
22 2 2 2
1
2(5 )(9.8)(.18)
.087 kg m4 4 (1.6)
mgdf
Imgd kg
If
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Problem 14.33
• The amplitude of an oscillator decreases to 36.8% of it initial value in 10.0s. What is the time constant?
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Problem 14.33
10.
( )
(0)
(10.0).368
(0)
10.ln(.368)
10.10.0
ln(.368)
t
MAX
MAX
MAX
MAX
x t Ae
x A
xe
x
s
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0 1 2 3 4 5 6 7 8
-4
-3
-2
-1
0
1
2
3
4
x(t) vs. t
x(t)
met
ers
2 4 6 8
The period of this oscillator is approximately
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The period of the oscillator is
1s 2s 5s 10s
25% 25%25%25%1. 1s2. 2s3. 5s4. 10s
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0 1 2 3 4 5 6 7 8
-4
-3
-2
-1
0
1
2
3
4
x(t) vs. t
x(t)
met
ers
2 4 6 8
The is zero at t = ?? approximately
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The velocity is zero when t =
1.25 s
2.60 s 5.2s
0.0 s
25% 25%25%25%1. 1.25 s2. 2.60 s3. 5.2s4. 0.0 s
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0 1 2 3 4 5 6 7 8
-4
-3
-2
-1
0
1
2
3
4
x(t) vs. t
x(t)
met
ers
2 4 6 8
The acceleration is a maximum when t = ??
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The acceleration is max when t=
0.00s
1.25 s 4.0 s
None of t
he above
25% 25%25%25%1. 0.00s2. 1.25 s3. 4.0 s4. None of the above
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0 1 2 3 4 5 6 7 8
-4
-3
-2
-1
0
1
2
3
4
x(t) vs. t
x(t)
met
ers
2 4 6 8
The velocity is a maximum for t = ??
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The velocity is a maximum for t =
0.0s 1.25s
2.6s 4.0s
25% 25%25%25%1. 0.0s2. 1.25s3. 2.6s4. 4.0s
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Problem 14. 37
• A 25 kg child sits on a 2.0m long rope swing. To maximize the amplitude of the swinging, how much time should be between pushes?
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Problem 14.37
2.02 2 2.84
9.8
LT s
g
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Problem 14.32
A thin, circular hoop with a radius of 0.22m is hanging from its rim on a nail. When pulled to one side and released, the hoop swings back and forth. The moment of inertia for a hoop with the axis passing through the circumference is I = 2MR2. What is the period of oscillation?
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Problem 14.32
2
2
2 22 2 1.33
IT
mgd
mR RT s
mgR g
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Exam IV Wednesday, December 3
Chapter 10 and 14Quick Review Monday