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IntroductoryPhysics
PHYS101
Dr RichardH.CyburtOfficeHours
TRF9:30-11:00am
AssistantProfessorofPhysics
F12:30-2:00pm
Myoffice:402cintheScienceBuilding
Myphone:(304)384-6006
Meetingsmayalsobearrangedatothertimes,
byappointment
Myemail:[email protected]
Inpersonoremailisthebestwaytogetahold
Checkmyscheduleonmyofficedoor.
ofme.
PHYS101
PHYS101:IntroductoryPhysics
400
Lecture:8:00-9:15am,TRScienceBuilding
Lab1:3:00-4:50pm,FScienceBuilding304
Lab2:1:30-3:20pm,MScienceBuilding304
Lab3:3:30-5:20pm,MScienceBuilding304
Lab20:6:00-7:50pm,MScienceBuilding304
PHYS101
MasteringPhysicsOnline
GotoHYPERLINK"http://www.masteringphysics.com."www.masteringphysics.com.
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PHYS101
Midterm1
Therewillbeasignoutsheetinmyoffice
◦ Youmustsigntogetyourexam
Therewillbeabonusassignment,basedonyourexam
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Willearnyouextrapointsonyourexam
Itwillbeonlineasahomework
Youmustdobetteronthisassignment,thanyourtesttogetbonuspoints
Bonus=30%x(Homework– Midterm)
BonusHomeworkisOnline,dueSep26,12:59pm(justbeforelabsstartfortheday)
PHYS101
Midterm2
Thursday,September29
CoveringChapters5-8
ReviewSession,Wednesday,September28,7:00-9:00pmS300
PHYS101
IntroductoryPhysics
PHYS101
DouglasAdams
Hitchhiker’sGuidetotheGalaxy
PHYS101
You’realreadyknowphysics!
Youjustdon’tnecessarilyknowtheterminologyand
languageweuse!!!
PhysicsofNASCAR
PhysicsofAngerBirds
PHYS101
Courtesyofmywife…..(andtheOED)
Yourwordfortodayis: noctambulate,v.
noctambulate, v.
[‘ intr. Towalkaboutatnight.’]
Pronunciation: Brit. /nɒkˈtambjᵿleɪt/, U.S. /nɑkˈtæmbjəˌleɪt/
Origin:Formed withinEnglish,bycompounding.Etymons: nocti- comb.form, ambulate v.
Etymology: < nocti- comb.form + ambulate v.,after noctambulation n., noctambulator n. Compare
French noctambuler (1866).Compareearliersomnambulate vb. at somn- comb.form .
intr. Towalkaboutatnight.1955 H.Spring TheseLoversfledAway 206 NowandthenIwould
noctambulate throughthecity.
1988 R.Johnson Oxf.Myths 21 RightupuntilthenineteenthcenturytheUniversitypolicecould
arrestcitizensfornoctambulating.
1993 A.Lane etal. SubterraneanWorld(song)in A.Lane DirtyPearl (recordsleevenotes), Allthe
bushybratsturnedoutfromThewesternburgs..Noctambulating aroundnowheresville.
PHYS101
Inclass!!
PHYS101
Thislecturewillhelpyouunderstand:
RecapofLecturespast
UsingLabasaPhysicsProblem
Questionsaboutforcesandcircularmotion
Newton’sLawofGravity
GravityandOrbits
PHYS101
ExercisewithDrag
Text:p.144
©2015PearsonEducation,Inc.
FromLab:Ropes&Pulleys
PHYS101
QuickCheck 6.14
Acoinsitsonaturntableasthetablesteadily
rotatescounterclockwise.Thefree-body
diagramsbelowshowthecoinfrom
behind,movingawayfromyou.Whichis
thecorrectdiagram?
©2015PearsonEducation,Inc.
QuickCheck 6.14
Acoinsitsonaturntableasthetablesteadily
rotatescounterclockwise.Thefree-body
diagramsbelowshowthecoinfrom
behind,movingawayfromyou.Whichis
thecorrectdiagram?
C.
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QuickCheck 6.15
Acarturnsacorneronabankedroad.
Whichofthediagramscould be
thecar’sfree-bodydiagram?
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QuickCheck 6.15
Acarturnsacorneronabankedroad.
Whichofthediagramscould be
thecar’sfree-bodydiagram?
E.
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Section6.5Newton’s
LawofGravity
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MassandWeight
Massandweightarenotthe
samething.
Massisaquantitythat
describesanobject’sinertia,
itstendencytoresistbeing
accelerated.
Weightisthegravitationalforceexertedonanobjectbyaplanet:
w =–mg
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GravityObeysanInverse-SquareLaw
Gravityisauniversal
forcethataffectsall
objectsintheuniverse.
Newtonproposedthat
theforceofgravity
hasthefollowing
properties:
1. Theforceisinverselyproportionaltothesquareofthedistancebetweentheobjects.
2. Theforceisdirectlyproportionaltotheproductofthemassesofthetwoobjects.
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GravityObeysanInverse-SquareLaw
Newton’slawofgravityisaninverse-squarelaw.
Doublingthedistancebetweentwomassescausestheforce
betweenthemtodecreasebyafactorof4.
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ConceptualExample6.11Varying
gravitationalforce
Thegravitationalforcebetweentwogiantleadspheresis0.010Nwhenthe
centersofthespheresare20mapart.Whatisthedistancebetweentheir
centerswhenthegravitationalforcebetweenthemis0.160N?
REASON Wecansolvethisproblemwithoutknowingthemassesofthetwo
spheres.Thekeyistoconsidertheratiosofforcesanddistances.Gravityisan
inverse-squarerelationship;theforceis
relatedtotheinversesquareofthedistance.Theforceincreases byafactorof
(0.160N)/(0.010N)= 16,sothedistancemustdecrease byafactorof= 4.
Thedistanceisthus(20m)/4= 5.0m.
ASSESS Thistypeofratioreasoningisaverygoodwaytogetaquickhandleonthe
solutiontoaproblem.
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Example6.12Gravitationalforce
betweentwopeople
Youareseatedinyourphysicsclassnexttoanotherstudent0.60maway.
Estimatethemagnitudeofthegravitationalforcebetweenyou.Assumethatyou
eachhaveamassof
65kg.
PREPARE Wewillmodeleachofyouasasphere.Thisisnotaparticularlygood
model,butitwilldoformakingan
estimate.Wewilltakethe0.60masthedistancebetweenyourcenters.
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Example6.12Gravitationalforce
betweentwopeople(cont.)
SOLVE ThegravitationalforceisgivenbyEquation6.15:
ASSESS Theforceisquitesmall,roughlytheweightofonehaironyourhead.This
seemsreasonable;youdon’tnormallysensethisattractiveforce!
©2015PearsonEducation,Inc.
QuickCheck 6.16
TheforceofPlanetYonPlanetXis___themagnitude
of.
◦ Onequarter
◦ Onehalf
◦ Thesameas
◦ Twice
◦ Fourtimes
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2M
M
Planet X
Planet Y
QuickCheck 6.16
TheforceofPlanetYonPlanetXis___themagnitude
of.
◦ Onequarter
◦ Onehalf
◦ Thesameas
◦ Twice
◦ Fourtimes
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2M
M
Newton’sthirdlaw
Planet X
Planet Y
QuickCheck 6.17
Thegravitationalforcebetweentwoasteroidsis
1,000,000N.Whatwilltheforcebeifthedistance
betweentheasteroidsisdoubled?
◦ 250,000N
◦ 500,000N
◦ 1,000,000N
◦ 2,000,000N
◦ 4,000,000N
©2015PearsonEducation,Inc.
QuickCheck 6.17
Thegravitationalforcebetweentwoasteroidsis
1,000,000N.Whatwilltheforcebeifthedistance
betweentheasteroidsisdoubled?
◦ 250,000N
◦ 500,000N
◦ 1,000,000N
◦ 2,000,000N
◦ 4,000,000N
©2015PearsonEducation,Inc.
GravityonOtherWorlds
Ifyoutraveledtoanotherplanet,yourmass wouldbethesamebutyourweight
wouldvary.Theweightofamassm onthemoonisgivenby
UsingNewton’slawofgravity(Eq.(6.15))theweightisgivenby
Sincethesearetwoexpressionsforthesameforce,theyareequaland
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GravityonOtherWorlds
Ifweusevaluesforthemassandtheradiusofthemoon,wecompute
gmoon = 1.62m/s2.
A70-kgastronautwearingan80-kgspacesuitwouldweighmorethan
330lb ontheearthbutonly54lb onthemoon.
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QuickCheck6.18
PlanetXhasfree-fallacceleration8m/s2 atthesurface.PlanetYhas
twicethemassandtwicetheradiusof
planetX.OnPlanetY
◦
◦
◦
◦
◦
g =2m/s2
g =4m/s2
g =8m/s2
g =16m/s2
g =32m/s2
©2015PearsonEducation,Inc.
QuickCheck6.18
PlanetXhasfree-fallacceleration8m/s2 atthesurface.PlanetYhas
twicethemassandtwicetheradiusof
planetX.OnPlanetY
◦
◦
◦
◦
◦
g =2m/s2
g =4m/s2
g =8m/s2
g =16m/s2
g =32m/s2
©2015PearsonEducation,Inc.
WeightlessnessinOrbit
Astronautsandtheirspacecraftareinfreefall.
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QuickCheck 6.19
AstronautsontheInternationalSpaceStationareweightless
because
◦ There’snogravityinouterspace.
◦ Thenetforceonthemiszero.
◦ Thecentrifugalforcebalancesthegravitationalforce.
◦ g isverysmall,althoughnotzero.
◦ Theyareinfreefall.
©2015PearsonEducation,Inc.
QuickCheck 6.19
AstronautsontheInternationalSpaceStationareweightless
because
◦ There’snogravityinouterspace.
◦ Thenetforceonthemiszero.
◦ Thecentrifugalforcebalancesthegravitationalforce.
◦ g isverysmall,althoughnotzero.
◦ Theyareinfreefall.
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Section6.6Gravityand
Orbits
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OrbitalMotion
Ifthelaunchspeedofa
projectileissufficientlylarge,
therecomesapointatwhich
thecurveofthetrajectoryand
thecurveoftheearthare
parallel.
Suchaclosedtrajectoryis
calledanorbit.
Anorbitingprojectileisin
freefall.
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OrbitalMotion
Theforceofgravityistheforcethatcausesthecentripetalaccelerationofan
orbitingobject:
Anobjectmovinginacircleofradiusr atspeedvorbit willhavethiscentripetal
accelerationif
Thatis,ifanobjectmovesparalleltothesurfacewiththespeed
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OrbitalMotion
Theorbitalspeedofaprojectilejustskimmingthesurfaceofasmooth,airless
earthis
Wecanusevorbit tocalculatetheperiodofthesatellite’sorbit:
©2015PearsonEducation,Inc.
QuickCheck6.22
A60-kgpersonstandsoneachofthefollowingplanets.
Onwhichplanetishisorherweightthegreatest?
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QuickCheck 6.22
A60-kgpersonstandsoneachofthefollowingplanets.
Onwhichplanetishisorherweightthegreatest?
A
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Example6.14Findingthespeedtoorbit
Deimos
Marshastwomoons,eachmuchsmallerthantheearth’smoon.
Thesmallerofthesetwobodies,Deimos,isn’tquitespherical,butwecanmodel
itasasphereofradius6.3km.
Itsmassis1.8× 1015 kg.
AtwhatspeedwouldaprojectilemoveinaveryloworbitaroundDeimos?
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Example6.14Findingthespeedtoorbit
Deimos(cont.)
SOLVE Thefree-fallaccelerationatthesurfaceofDeimos issmall:
©2015PearsonEducation,Inc.
Example6.14Findingthespeedtoorbit
Deimos(cont.)
Giventhis,wecanuseEquation6.13tocalculatetheorbitalspeed:
ASSESS Thisisquiteslow.Withagoodjump,youcouldeasilylaunchyourselfintoanorbitaround
Deimos!
©2015PearsonEducation,Inc.
GravityandOrbits
Newton’ssecondlawtells
usthatFM onm = ma,where
FM onm isthegravitational
forceofthelargebody
onthesatelliteanda is
thesatellite’sacceleration.
Becauseit’smovingina
circularorbit,Newton’s
secondlawgives
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GravityandOrbits
Asatellitemusthavethis
specificspeedinorderto
maintainacircularorbit
ofradiusr aboutthelarger
massM.
©2015PearsonEducation,Inc.
GravityandOrbits
Foraplanetorbitingthesun,theperiodT isthetimetocompleteonefullorbit.
Therelationshipamongspeed,radius,andperiodisthesameasforanycircular
motion:
v =2πr/T
Combiningthiswiththevalueofv foracircularorbitfromEquation6.21gives
Ifwesquarebothsidesandrearrange,wefindtheperiodofasatellite:
©2015PearsonEducation,Inc.
QuickCheck 6.20
Twosatelliteshavecircularorbitswiththesameradius.Whichhasa
higherspeed?
◦ Theonewithmoremass.
◦ Theonewithlessmass.
◦ Theyhavethesamespeed.
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QuickCheck 6.20
Twosatelliteshavecircularorbitswiththesameradius.Whichhasa
higherspeed?
◦ Theonewithmoremass.
◦ Theonewithlessmass.
◦ Theyhavethesamespeed.
©2015PearsonEducation,Inc.
QuickCheck 6.21
Twoidenticalsatelliteshavedifferentcircularorbits.Whichhasa
higherspeed?
◦ Theoneinthelargerorbit
◦ Theoneinthesmallerorbit
◦ Theyhavethesamespeed.
©2015PearsonEducation,Inc.
QuickCheck 6.21
Twoidenticalsatelliteshavedifferentcircularorbits.Whichhasa
higherspeed?
◦ Theoneinthelargerorbit
◦ Theoneinthesmallerorbit
◦ Theyhavethesamespeed.
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QuickCheck6.23
Asatelliteorbitstheearth.ASpaceShuttlecrewissenttoboostthe
satelliteintoahigherorbit.Whichofthesequantitiesincreases?
◦ Speed
◦ Angularspeed
◦ Period
◦ Centripetalacceleration
◦ Gravitationalforceoftheearth
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QuickCheck6.23
Asatelliteorbitstheearth.ASpaceShuttlecrewissenttoboostthe
satelliteintoahigherorbit.Whichofthesequantitiesincreases?
◦ Speed
◦ Angularspeed
◦ Period
◦ Centripetalacceleration
◦ Gravitationalforceoftheearth
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Example6.15Locatingageostationary
satellite
Communicationsatellitesappearto“hover”overonepointontheearth’s
equator.Asatellitethatappearstoremainstationaryastheearthrotatesissaid
tobeinageostationaryorbit.Whatistheradiusoftheorbitofsuchasatellite?
PREPARE Forthesatellitetoremainstationarywithrespecttotheearth,the
satellite’sorbitalperiodmustbe24hours;insecondsthisisT= 8.64× 104 s.
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Example6.15Locatingageostationary
satellite(cont.)
SOLVE WesolvefortheradiusoftheorbitbyrearrangingEquation6.22.Themass
atthecenteroftheorbitistheearth:
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Example6.15Locatingageostationary
satellite(cont.)
ASSESS Thisisahighorbit,andtheradiusisabout7timestheradiusoftheearth.
RecallthattheradiusoftheInternationalSpaceStation’sorbitisonlyabout5%
largerthanthatoftheearth.
©2015PearsonEducation,Inc.
GravityonaGrandScale
Nomatterhowfaraparttwoobjectsmaybe,thereisagravitationalattraction
betweenthem.
Galaxiesareheldtogetherbygravity.
Allofthestarsinagalaxyaredifferentdistancesfromthegalaxy’scenter,andso
orbitwithdifferentperiods.
©2015PearsonEducation,Inc.
ExampleProblem
Phobos isthecloserofMars’stwosmallmoons,orbiting
at9400kmfromthecenterofMars,aplanetofmass
6.4× 1023kg.
WhatisPhobos’s orbitalperiod?
HowdoesthiscomparetothelengthoftheMartianday,whichisjust
shyof25hours?
©2015PearsonEducation,Inc.