lecture 7 chapter 10,11 - university of...
TRANSCRIPT
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Lecture 7�Chapter 10,11�
Rotation, Inertia, Rolling, �
Torque, and �Angular momentum
Demo
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Demos
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Summary of Concepts to Cover from chapter 10 Rotation
• Rotating cylinder with string wrapped around it: example • Kinematic variables analogous to linear motion for constant
acceleration • Kinetic energy of rotation • Rotational inertia • Moment of Inertia • Parallel axis theorem
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ROTATION ABOUT A FIXED AXIS • Spin an rigid object and define rotation axis. • Define angular displacement, angular velocity and angular
acceleration. • Show how angle is measured positive (counterclockwise). • Interpret signs of angular velocity and acceleration. • Point out analogy to 1D motion for the variables. • Point out that omega and alpha are vectors that lie along
the axis of rotation for a fixed axis of rotation Angular displacements are not vectors. Show the figure with two angular displacements of a book.
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ROTATION WITH CONSTANT ANGULAR ACCELERATION �
• Restrict discussion to a fixed axis of rotation but also applies if the axis is in translation as well.
• Write down or point out the analogy of the angular kinematic equations with linear motion. See Table 11.1 in text
• Same strategies are used to solve rotational problems as linear ones.
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Rotation with constant angular acceleration: Consider some string wound around a cylinder.�There is no slippage between string and cylinder.� Red dot indicates a spot on the cylinder �that is rotating as I apply a force to the massless string
θ
v
s r
mg
T ma
r . Front view
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ω
θ
v
s r
Red dot indicates a spot on the cylinder �that is rotating as I apply a force to the massless string
Front view Isometric view
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Define radians
Conversion from degrees to radians is 0.0174 radians per degree or 57.3 degrees per radian
For θ =360 degrees
s and θ are not vectors
q
v
s r θ =sr=
2πrr
= 2π radians in a circle
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Define angular velocity
Take derivative holding r constant
θ
v
s r ω
s = rθ
dsdt
= rdθdt
v =dsdt
ω =dθdt
v = rω v=ω× r
where ω is in radians per secand θ is in radians
Angular velocity: Vector Magnitude Direction Units
Tangential velocity
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Use Right hand rule to get direction of w
ω
Counterclockwise is + for angular displacement θ and angular velocity ω.
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Recall there is also the radial acc.
Also called the tangential acceleration
α is in the same or opposite direction as ω
v = rω
at =dvdt
= rdωdt
at = αrα =
dωdt(rad / s2 )
ar =v2
r=ω 2r
Define Angular or Rotational Acceleration
is called the angular acceleration
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Two Kinds of Acceleration Tangential acceleration
Radial acceleration
in radial direction all the time
are perpendicular to each other
v
w
at
ar
a
at = αr =dωdt
r
arv2
r=ω 2r
a= at + ar
at + ar
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For constant acceleration
�
x− x0 = 12(v0 + v)t
x− x0 = v0t+ 12 at
2
v = v0 + at
v2 = v02 + 2a(x− x0 )
�
θ −θ 0 = 12(ω0 +ω )t
θ −θ 0 = ω0t+ 12αt
2
ω = ω0 +αt
ω 2 = ω02 + 2α(θ −θ 0 )
We have an analogous set of formulas for angular variables
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What is the acceleration of the mass?�How do we take into account the �
rotation of the pulley?
ω
Need more information
θ
v
s r
mg
T ma ∑Fy = T − mg = −ma
Free body diagram
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How do we define kinetic energy of a rotating body?�Kinetic energy of rotation and rotational inertia I.
Looks strange
Consider what is the kinetic energy of a small rigid object moving in a circle?
r
ar
v
θ We call mr2 the moment of inertia I.
Kinetic energy of rotation
It is very important that we can define such a variable I. Here is
why
K =12mv2
v =ωr
K =12mr2ω 2
K =12Iω 2
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Suppose our rotating body is a rigid rod. Now how do we define the kinetic energy?
ri
v
s θ
w
�
K = 12Iω 2
�
I = ri2
i=1
imax
∑ Δmi =
�
r20
M∫ dm
�
= 13 ML
2
�
K = 16ML2ω 2
I is called the moment of inertia about an axis through the end of the rod.
K =12mr2ω 2
K =12∑miri
2ω 2
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Evaluation of the rotational inertia
�
r20
M∫ dm
�
= ρ 13 r
30L = ρ L
3
3= MLL3
3= 1
3 ML2
for a rod rotating about an axis through the end perpendicular to the length
= I
θ
L
v
.
�
I = ri2
i=1
imax
∑ Δmi =
�
= ρ r20
L∫ dr
dm = ρdr
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Now consider rod rotating about an axis through the center of mass of the rod
q L
v
.
�
r20
M∫ dm = 2ρ r20
L2∫ dr = 2ρ 1
3 r30
L2 = 2 M
L13L3
8= 112 ML
2Icom =
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Parallel Axis Theorem
q L
v
.
Notice that the difference
r
com
General relation
Iend − Icom =13ML2 −
112
ML2 =14ML2 = M (L
2)2 = Mr2
Iend = Icom + Mr2
I p = Icom + Mr2
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I p = Icom + Mh2
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Moment of inertia of long thin rod of same radius and length rotating about different
axes. L
R I =12MR2
I =112
ML2 I =13ML2
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Parallel Axis Theorem
Icom =112
ML2
I =112
ML2 + M (L2)2 = 1
12ML2 +
14ML2 =
13ML2
I p = Icom + Mh2
L L
h=L/2
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MOMENT OF INERIA FOR A PULLEY
Rotating about central axis
Demo for moment of inertia Rotate hoops and cylinders down an incline plane
I =12Mr2
θ
v
s r
mg
T ma Still need more information to find T
∑Fy = T − mg = −ma
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Concepts in Chapter 11�Rolling, Torque, and Angular Momentum
• Torque • Newton’s Law for rotations • Work Energy theorem for rotations • Atwood machine with mass of pulley • More on rolling without sliding • Center of percussion: baseball bat and hammer • Sphere rolling down an inclined plane • Yo -yo • Angular momentum • Conservation of angular momentum • Breaking a stick balanced on a wine glass
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Torque. It is similar to force but it also depends on the axis of rotation. Why do we
have to define torque?
• Use towel to open stuck cap on jar
• Door knob far from hinge • Screw driver with large fat
handle • Lug wrench to unscrew
nuts on rim for tires
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Torque
• Also Torque = • r is a vector perpendicular to the rotation axis. f is the angle between r and F when the tails are together. • If F is along r, the torque is 0. Only component of F that is
perpendicular to r contributes to torque or at. Parallel component contributes to ar.
• Increase r or F to get more torque. • Positive torque corresponds to counterclockwise rotation • Long handled wrench, door knob far from the hinge.
Torque = rF sinφ = r × F = τrperpF = rFt
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Torque = rF sinφ = r × F = τ
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Torque = rF sinφ = r × F = τ
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Newton’s 2nd law for rotation
Suppose I have small mass at the end of a massless rod
τ net = Iα
Ft = matrFt = mrat
=mr2atr
= Iα
I = mr2
α =atr
rFt = rF sinφr ×F = Iα
τ = Iα
Consider the consistency argument below
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What is the acceleration of the mass?�Now we can take into account the �
rotation of the pulley?
θ
v
r
mg
T ma
Mp
r = radiusF = T
I =12Mpr
2
α = a r
∑Fy = T − mg = −mar ×F = I
α
rT =12Mpr
2 ar
T =Mpa2
Ma2
− mg = −ma
a =2mg
(Mp + 2m)
T =Mpm
(Mp + 2m)
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Frictionless Sideways Atwood machine with a pulley with mass
Now take into account the rotation of the pulley.
a
new equation
T2
T1 Ia = (T2-T1) R
+x
-y T1 = MaT2 − mg = −maIα = (T2 − T1)Ra = αR12MpR
2 aR= (T2 − T1)R
12MpR
2 aR= (mg − ma − Ma)R
a =m
(m + M +Mp
2)g
Mp
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Now include friction between block M and surface
a
new equation
T2
T1 Ia = (T2-T1) R
+x
-y T1 − µMg = MaT2 − mg = −ma
Iα = (T2 − T1)Ra = αR12MpR
2 aR= (T2 − T1)R
12MpR
2 aR= (mg − ma − Ma − µMg)R
a =m(1− µ)
(m + M +Mp
2)g
Mp
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L = I
ω
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Inclined plane rolling demo.�Which object gets to the bottom
first and why?
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Case I: Frictionless plane. Pure translation, No rotation. Then K =1/2 MV2
at the bottom of the plane
Now we want to understand why objects accelerated at different rates down the inclined plane. What is its total kinetic energy of the object
at the bottom of the inclined plane?
h
V
V = 0
Case II: Slipless plane, Translation and rolling down inclined plane. Then your guess might be and you would be right. K =
12MVcom
2 +12Icomω
2
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From s=θR we get ds/dt = dθ/dt R or vcom = ω R for smooth rolling motion
Linear speed of the center of mass of wheel is ds/dt
The angular speed ω about the com is dθ/dt.
without slipping
First we have to ask what is rolling without slipping?
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Rolling can be considered rotating about an axis through the com while the center of mass moves.
At the bottom P is instantaneously at rest. The wheel also moves slower at the bottom because pure rotation motion and pure translation partially cancel out See photo in Fig 12-4. Also tire tracks are clear in the snow and are not smudged.
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Rolling as pure rotation Consider rolling as pure rotation with angular velocity w about an axis passing through point P. The linear speed at the top is vT = ω(2R) = 2 vcom ( same as before)
The kinetic energy now is What is IP? K =
12I pω
2
I p = Icom + MR2
K =12Icomω
2 +12MR2ω 2
=12Icomω
2 +12Mvcom
2
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What is the acceleration of a sphere smoothly rolling down an inclined plane?
a) Drop an object?
b) Block sliding down a frictionless inclined plane?
c) With friction?
d) Sphere rolling down an inclined plane? ????
�
acom =
a = −g
a = −gsinθ
a = −g(sinθ − µ cosθ)
Negative x
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What is the acceleration of a sphere smoothly rolling down an inclined plane?
x component Newtons Law
Fnet = Macom
fs − Mgsinθ = Macom
τnet= Iα
Solve for fs
Rfs = Icomα
α =−acomR
Find torque about the com Negative x
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�
acom = − gsinθ
1+ IcomMR2
This will predict which objects will roll down the inclined faster.
fs = Icomα / R
α =−acomR
fs = −Icomacom / R2
−Icomacom / R2 − Mgsinθ = Macom
Solve for
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shape Icom 1+ Icom/MR2 acom
sphere 2/5 MR2 1.4 0.71x g/2
disk 1/2MR2 1.5 0.67 x g/2
pipe MR2 2.0 0.50 x g/2
�
acom = − gsinθ
1+ IcomMR2
Sin 30 = 0.5 Let θ= 30 deg
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�
acom = − gsinθ
1+ IcomMR2
Yo-yo rolls down the string as if it were inclined plane at 90 degrees
Instead of friction, tension in the string holds it back
The moment of inertia Icom is that of the yo-yo itself. �
acom = − g
1+ IcomMR0
2
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Can objects fall with a greater acceleration than gravity? Work out with in class.
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Things to consider
• Angular momentum of a rigid body
• Conservation of angular momentum
L = r × p = I
ω
Iiω i = I fω f
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Another Stick Problem A Stick resting on ice is struck at one end with an impulsive force F perpendicular to its length
The stick has mass M and length L
(a) Find the acceleration of the center of mass (CM) (b) Find the angular velocity of the stick about the CM (c) Find the velocity of each end of the stick in terms of the velocity of the CM (d) Find the center of percussion (CP), which is where the rotational and translational motion cancel.
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Diagram
F
CM
. x
VRE
VLE
VCM
CP
Δt is the time over which the force lasted
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(a) F
CM
.
VRE
VLE
VCM
Find the acceleration of the center of mass (CM) F = MaCM
aCM =FM
⇒ F =ΔPΔt
=M (VCM − 0)
Δt=MVCMΔt
aCM =MVCMMΔt
aCM =VCMΔt
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(b)
Use and conservation of angular momentum
L = Iω = r × p
ΔL = Iω = r × Δp
I =112
ML2 =
Δp = MVCM =
r =L2=
Moment of inertia about center of stick
Just after the blow
Moment arm
112
ML2ω =L2MVCM
ω =6VCML
Find the angular velocity of the stick about the CM
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(c) Find the velocity of each end of the stick in terms of the velocity of the CM
VRE = VCM +L2ω
VRE = VCM +L2(6VCML)
VRE = VCM + 3VCM
VRE = 4VCM
VLE = VCM −L2ω
VLE = VCM −L2(6VCML)
VLE = VCM − 3VCM
VLE = −2VCM
(d) Find the center of percussion (CP), which is where the rotational and translational motion cancel
V (x) = VCM − xω = VCM − x(6VCML) = 0
x =L6
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The Breaking Broomstick Demo (1)
(2)
(3)
(4)
(5)
(6)
(7)
“Experiment to demonstrate Inertia”
First published in 1881
Dramatic-Why does the stick break so violently and leave the glass intact?
Results somewhat counterintuitive
Example of Newton’s First Law
Notion of Impulse
Modeling
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Physics Points (1)
(2)
(3)
(4)
(5)
(6)
(7)
Motion of each half behaves similarly to a single rod receiving A blow at one end. Observe end of rod. What happens?
Center of mass almost falls in a straight line under gravity.
Observe point on stick 2/3 from CM where the rotational Motion is canceled by the vertical motion.(Instantaneous axis of rotation)
Slight sideways constant velocity due to blow
Measure and compare to simple model
Estimate and deduce F. Δt
vLE ,vCM ,vRE
vLE = −2vCMvRE = 4vCM
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Breaking Broomstick Demo vLE
vCM
2FΔt
(1)
(2)
vCM =FΔtm
Iω =L2(FΔt) − τΔt[ ]
Iω =L2mvCM − τΔt[ ]
TOTAL MASS: TOTAL LENGTH:
Resistive torque
vLE = vCM −L2ω
2m2L
L =
L2
(3)
Iω = r × p
Angular momentum about L/2
(Moment taken about VCM
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Broomstick Breaking Cont. vLE
vCM
2FΔt
Resistive torque
LEFT SIDE RIGHT SIDE
vLE = vCM −L2(L2
mvCM112mL
2) − −
τΔt112mL
2
⎡
⎣⎢⎢
⎤
⎦⎥⎥
vLE = vCM − 3vCM +6τΔtmL
vLE = −2vCM +6τΔtmL
vLE = −2vCM
vRE = vCM +L2ω
vRE = vCM + 3vCM
vRE = 4vCM
vRE = 4vCM −6τΔtmL
w/out resistive torque w/out resistive torque
w/ resistive torque w/ resistive torque
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Apparatus - 4 ft long, 7/8in diameter white pine, cedar, or hickory dowel rod or broomstick
-Stick pins in each end; cut off heads
-Support each pin with a wine glass, coke can, block of wood, etc.
-Striking stick: Steel 1/2” in diameter and 2ft long
-Mark the halfway point of stick so you know where to strike it
-Use a hacksaw to etch it around the circumference; avoid stick fracturing due to other weakness.
-Raise striking stick and hit the center as hard as you can; follow through
Need clearance
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Torque: magnitude and direction
T = r ×
F
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Angular momentum :�magnitude and direction
L = r × p = I
ω
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Work-energy theorem for rotations
W =12Iω f
2 −12Iω i
2
W = τ (θ f −θi )P = τω
τ = r ×
F
L = r × p
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Conservation of Angular momentum
Iω i = I
ω f
Sit on stool with dumbells held straight out 180 degrees apart. Slowly rotate the stool then bring arms in towards body. Angular velocity will increase drastically.
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Spin up bicycle wheel on buffer and hold its axis vertical while sitting on the stool. Slowly rotate the wheel axis 180 degrees and the stool will rotate in accordance with the law of conservation of angular momentum.
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Precession of a Bicycle Wheel Gyroscope Under Gravitational Force
In each of the lecture halls there is a string which hangs from the ceiling and has a hook attached to its free end. Spin the bicycle wheel up with the buffer wheel and attach the axle of the wheel to the hook on the string. With the axis of the bicycle wheel held horizontal, it is released. The axis remains horizontal while the wheel precesses around a vertical axis.
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ConcepTest 9.1b Bonnie and Klyde II
w
Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one revolution every two seconds. Who has the larger linear (tangential) velocity?
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1) 1/2 θ 2) 1/4 θ 3) 3/4 θ 4) 2 θ 5) 4 θ
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ConcepTest 9.4 Using a Wrench You are using a wrench to
loosen a rusty nut. Which
arrangement will be the most effective in loosening the nut?
1
3 4
2
5) all are equally effective
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ConcepTest 9.9 Moment of Inertia
same mass & radius
solid hollow
Two spheres have the same radius and equal masses. One is made of solid aluminum, and the other is made from a hollow shell of gold.
Which one has the bigger moment of inertia about an axis through its center?
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ConcepTest 9.10 Figure Skater A figure skater spins with her arms
extended. When she pulls in her arms, she reduces her rotational inertia and spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be
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w
BonnieKlyde V21V =
Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-go-round makes one revolution every two seconds. Who has the larger linear (tangential) velocity?
ConcepTest 9.1b Bonnie and Klyde II
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1) 1/2 2) 1/4 3) 3/4 4) 2 5) 4
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ConcepTest 9.4 Using a Wrench You are using a wrench to
loosen a rusty nut. Which arrangement will be the most effective in loosening the nut?
1
3 4
2
5) all are equally effective
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ConcepTest 9.9 Moment of Inertia
same mass & radius
solid hollow
Two spheres have the same radius and equal masses. One is made of solid aluminum, and the other is made from a hollow shell of gold.
Which one has the bigger moment of inertia about an axis through its center?
Moment of inertia depends on mass and distance from axis squared. It is bigger for the shell since its mass is located farther from the center.
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ConcepTest 9.10 Figure Skater A figure skater spins with her arms
extended. When she pulls in her arms, she reduces her rotational inertia and spins faster so that her angular momentum is conserved. Compared to her initial rotational kinetic energy, her rotational kinetic energy after she pulls in her arms must be
KErot=1/2 I 2 = 1/2 L (used L= I ). Since L is conserved, larger means larger KErot. The “extra” energy comes from the work she does on her arms.