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PHY 113 C Fall 2013 -- Lecture 17 110/24/2013
PHY 113 C General Physics I11 AM – 12:15 PM TR Olin 101
Plan for Lecture 17:Review of Chapters 9-13, 15-16
1. Comment on exam and advice for preparation
2. Review3. Example problems
PHY 113 C Fall 2013 -- Lecture 17 210/24/2013
PHY 113 C Fall 2013 -- Lecture 17 310/24/2013
Webassign questions – Assignment #15
Consider the sinusoidal wave of the figure below with the wave function y = 0.150 cos(15.7x − 50.3t)where x and y are in meters and t is in seconds. At a certain instant, let point A be at the origin and point B be the closest point to A along the x axis where the wave is 43.0° out of phase with A. What is the coordinate of B?
xoo 7.15
18043
PHY 113 C Fall 2013 -- Lecture 17 410/24/2013
Webassign questions – Assignment #15
A transverse wave on a string is described by the following wave function. y = 0.115 sin ((π/9)x + 5πt)where x and y are in meters and t is in seconds.
(a) Determine the transverse speed at t = 0.150 s for an element of the string located at x = 1.50 m.
(b) Determine the transverse acceleration at t = 0.150 s for an element of the string located at x = 1.50 m.
tx
ttxy 5
9cos
9115.0),(
tx
ttxy 5
9sin
9115.0),( 2
2
2
PHY 113 C Fall 2013 -- Lecture 17 510/24/2013
Webassign questions – Assignment #15
A sinusoidal wave in a rope is described by the wave function y = 0.20 sin (0.69πx + 20πt)where x and y are in meters and t is in seconds. The rope has a linear mass density of 0.230 kg/m. The tension in the rope is provided by an arrangement like the one illustrated in the figure below. What is the mass of the suspended object?
mgTk
c 69.0
20
tkxy sin0
T
mg
PHY 113 C Fall 2013 -- Lecture 17 610/24/2013
Comment about exam on Tuesday 10/29/2013
PHY 113 C Fall 2013 -- Lecture 17 710/24/2013
iclicker questionWhat is the purpose of exams?
A. Pure pain and suffering for all involved.B. To measure what has been learned.C. To help students learn the material.D. Other.
PHY 113 C Fall 2013 -- Lecture 17 810/24/2013
Advice on how to prepare for the exam
Review lecture notes and text chapters 9-13,15-16 Prepare equation sheetWork practice problems
Topics covered
Linear momentumRotational motion and angular momentumGravitational force and circular orbitsStatic equilibriumSimple harmonic motionWave motion
PHY 113 C Fall 2013 -- Lecture 17 910/24/2013
What to bring to exam:
Clear headCalculatorEquation sheetPencil or pen
PHY 113 C Fall 2013 -- Lecture 17 1010/24/2013
iclicker question:Have you looked at last year’s exams? A. Yes B. No
PHY 113 C Fall 2013 -- Lecture 17 1110/24/2013
Linear momentum What is it? When is it “conserved”? Conservation of momentum in analysis of collisions Notion of center of mass
dtd
dtmd
dtdmm
m
pvvaF
pv
sNm/skg :momentumlinear of Units :momentum"linear " Define
if
f
i
f
i
ddt
ddt
pppFI
pF
:Impulse
PHY 113 C Fall 2013 -- Lecture 17 1210/24/2013
Linear momentum -- continued
Physics of composite systems
ii
i
ii
i
ii
iii
ii dt
ddtmd
dtdmm pvvaF
:law second sNewton'
ii
ii
ii
ii
ii
dtd
final initial
(constant)
0
: then,0 if that Note
pp
p
p
F
PHY 113 C Fall 2013 -- Lecture 17 1310/24/2013
Example – completely inelastic collision; balls moving in one dimension on a frictionless surface
i
iiv
iviv
vvv
vvv
pp
ˆ 0.125
/5.03.0
ˆ15.0ˆ23.0
ˆ/1 ,ˆ/2
5.0 ,3.0For
21
21
21
2211
212211
final initial
m/s
sm
smsm
kgmkgmmmmm
mmmm
f
ii
iif
fii
ii
ii
PHY 113 C Fall 2013 -- Lecture 17 1410/24/2013
Examples of two-dimensional collision; balls moving on a frictionless surface
-- equations more 2 Need
,,, :Unknowns,, :Knowns
sinsin0
coscos
21
121
2211
221111
final initial
ff
i
ff
ffi
ii
ii
vvvmm
vmvm
vmvmvm
pp
PHY 113 C Fall 2013 -- Lecture 17 1510/24/2013
The notion of the center of mass and the physics of composite systems
2
2
2
2
2
2
:Define
:law second sNewton'
dtdM
mMM
mdtmd
dtdmm
CMtotal
ii
ii
iii
CM
i
ii
i
ii
iii
ii
rFF
rr
rraF
PHY 113 C Fall 2013 -- Lecture 17 1610/24/2013
Finding the center of mass
ii
iii
CM mMM
m
rr
ji
jiir
jiir
ˆ00.1ˆ750 4
ˆ22ˆ21ˆ)1)(1(
ˆˆˆ2 ;1 :example In this
321
332211
321
mm.
mmm
mmmymxmxm
kgmkgmm
CM
CM
PHY 113 C Fall 2013 -- Lecture 17 1710/24/2013
Rotational motion and angular momentum Angular variables Newton’s law for angular motion Rotational energy Moment of inertia Angular momentum
dtddtd
PHY 113 C Fall 2013 -- Lecture 17 1810/24/2013
Review of rotational energy associated with a rigid body
iii
iii
iii
iiirot
rmI
Irm
rmvmK
2
222
22
here w21
21
21
21
:energy Rotational
PHY 113 C Fall 2013 -- Lecture 17 1910/24/2013
Moment of inertia: i
iirmI 2
22MaI 22 22 mbMaI
PHY 113 C Fall 2013 -- Lecture 17 2010/24/2013
22
21
21
:object rolling ofenergy kinetic Total
CM
CMrollingtotal
MvI
KKK
CMvRdtdR
dtds
dtd
: thatNote
22
222
21
21
21
CM
CM
CMrollingtotal
vMRI
MvRRI
KKK
22
21
21
:object rolling ofenergy kinetic Total
CM
CMrollingtotal
MvI
KKK
CMvRdtdR
dtds
dtd
: thatNote
22
222
21
21
21
CM
CM
CMrollingtotal
vMRI
MvRRI
KKK
CMCM
PHY 113 C Fall 2013 -- Lecture 17 2110/24/2013
iclicker exercise:Three round balls, each having a mass M and radius R, start from rest at the top of the incline. After they are released, they roll without slipping down the incline. Which ball will reach the bottom first?
AB C
2MRI A 22 5.0
21 MRMRIB
22 4.052 MRMRIC
2
22
/12
01210
MRIghv
vMR
IMMgh
UKUK
CM
CM
ffii
PHY 113 C Fall 2013 -- Lecture 17 2210/24/2013
How can you make objects rotate?
Define torque:
t = r x F
t = rF sin
r
F
αarτFraF
Imm
sinr
F sin
ατ I:motion rotationalfor law sNewton'
PHY 113 C Fall 2013 -- Lecture 17 2310/24/2013
Example form Webassign #11
X
t1
t3
t2
iclicker exerciseWhen the pivot point is O, which torque is zero?
A. t1?B. t2?C. t3?
PHY 113 C Fall 2013 -- Lecture 17 2410/24/2013
Vector cross product; right hand rule
sinBACBAC
ˆ ˆ ˆ ˆ ˆ ˆ 0ˆ ˆ ˆ ˆ ˆ
ˆ ˆ ˆ ˆ ˆ
ˆ ˆ ˆ ˆ ˆ
i i j j k ki j j i kj k k j ik i i k j
PHY 113 C Fall 2013 -- Lecture 17 2510/24/2013
From Newton’s second law – continued – conservation of angular momentum:
(constant)
0 0 If
:Define
L
Lτ
prL
prτFr
dtd
dtd
PHY 113 C Fall 2013 -- Lecture 17 2610/24/2013
Example of conservation of angular momentum
wheelbf
wheelwheelbf
wheelibiwheelfbf
LL
LLL
LLLL
2
0
PHY 113 C Fall 2013 -- Lecture 17 2710/24/2013
Summary – conservation laws we have studied so far
Conserved quantity Necessary conditionLinear momentum p Fnet = 0Angular momentum L tnet = 0
Mechanical energy E No dissipative forces
PHY 113 C Fall 2013 -- Lecture 17 2810/24/2013
Fundamental gravitational force law and planetary motion Newton’s gravitational force law Gravity at Earth’s surface Circular orbits of gravitational bodies Energy associated with gravitation and orbital motion
PHY 113 C Fall 2013 -- Lecture 17 2910/24/2013
Universal law of gravitation Newton (with help from Galileo, Kepler, etc.) 1687
212
122112
ˆrmGm rF
2
211
kgmN 10674.6
G
PHY 113 C Fall 2013 -- Lecture 17 3010/24/2013
Gravitational force of the Earth
RE m
2226
2411
2
2
m/s8.9m/s)1037.6(
1098.51067.6
E
E
E
E
RGMg
RmGMF
Note: Earth’s gravity acts as a point mass located at the Earth’s center.
PHY 113 C Fall 2013 -- Lecture 17 3110/24/2013
days 27.4 s 32367353.951098.51067.6
)1084.3(π2
π2
π2ω
:Earth todueMoon for law sNewton'
2411
38
3
2
2
E
EM
EMEM
EM
E
EM
MMM
GMRT
RT
Rv
RGM
Rva
M aF
Stable circular orbit of two gravitationally attracted objects (such as the moon and the Earth)
REM
F
a
v
PHY 113 C Fall 2013 -- Lecture 17 3210/24/2013
m1
R2
R1
m2
v1
v2
Circular orbital motion about center of mass
CM
21
321
21
2
111
1
2
1
11
1
21
1
2211
2
22
2221
21
1
21
1
2
212
mmGRRTT
TRm
RTRm
Rvm
RmRmRvm
RRmGm
Rvm
2
31
21
321
21
1212
22
then if that Note
GmR
mmGRRTT
RRmm
PHY 113 C Fall 2013 -- Lecture 17 3310/24/2013
(const)
0
L
Lτdtd
m1
R2
R1
m2
v1
v2
L1=m1v1R1
L2=m2v2R2
L = L1 + L2
2
2
1
1
RL
RL
Note: More generally, stable orbits can be elliptical.
PHY 113 C Fall 2013 -- Lecture 17 3410/24/2013
Gravitational potential energy
rmGm dr
rmGmrU
rmGmdrU
r
gravity
r
rgravity
ref
212
21
221
''
)(
ˆ )(
rFrF
hRmGMhRrU
E
SEEgravity
)(
Example:
PHY 113 C Fall 2013 -- Lecture 17 3510/24/2013
Analysis of static equilibrium
Meanwhile – back on the surface of the Earth:
Conditions for stable equilibrium
0 : torqueof Balance
0 :force of Balance
ii
ii
τ
F
PHY 113 C Fall 2013 -- Lecture 17 3610/24/2013
0)()2( :Torques 1 CMg RmgmF
PHY 113 C Fall 2013 -- Lecture 17 3710/24/2013
T
Mgmg
**X
2/
x
t
sin2//
0sin2
0
MgmgxT
TMgmgx
NTNMgNmgmxmo
313200 600
2 8 53For
PHY 113 C Fall 2013 -- Lecture 17 3810/24/2013
Some practice problems
PHY 113 C Fall 2013 -- Lecture 17 3910/24/2013
PHY 113 C Fall 2013 -- Lecture 17 4010/24/2013
From webassign:
A 100-kg merry-go-round in the shape of a uniform, solid, horizontal disk of radius 1.50 m is set in motion by wrapping a rope about the rim of the disk and pulling on the rope. What constant force would have to be exerted on the rope to bring the merry-go-round from rest to an angular speed of 0.800 rev/s in 2.00 s? (State the magnitude of the force.)
FR
view from top:
2
21 MRI
I
αFrτ
PHY 113 C Fall 2013 -- Lecture 17 4110/24/2013
From webassign:A 10.3-kg monkey climbs a uniform ladder with weight w = 1.24 102 N and length L = 3.35 m as shown in the figure below. The ladder rests against the wall and makes an angle of θ = 60.0° with the ground. The upper and lower ends of the ladder rest on frictionless surfaces. The lower end is connected to the wall by a horizontal rope that is frayed and can support a maximum tension of only 80.0 N.