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IntroductoryPhysics

PHYS101

Dr RichardH.CyburtOfficeHoursAssistantProfessorofPhysics

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PHYS101

PHYS101:IntroductoryPhysicsLecture:8:00-9:15am,TRScienceBuilding400Lab1:3:00-4:50pm,FScienceBuilding304

Lab2:1:30-3:20pm,MScienceBuilding304



NoLab:Fri,Sep2:Extendedofficehoursin304,3:00-4:50pm

PHYS101

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PHYS101

Midterm1ReviewSession:Wednesday,Sept.7,7:00-9:00pmScienceBuilding300.

Midterm:Thursday,Sept.8,8:00-9:15amScienceBuilding400.

Chapters1-4,Notcircularmotion

PHYS101

IntroductoryPhysics

PHYS101

PHYS101

DouglasAdamsHitchhiker’sGuidetotheGalaxy

You’realreadyknowphysics!Youjustdon’tnecessarilyknowtheterminologyandlanguageweuse!!!

PhysicsofNASCARPhysicsofAngerBirds

PHYS101

Inclass!!

PHYS101

Thislecturewillhelpyouunderstand:SolvingProjectileMotionProblemsMotionandForces(Newton’sFirstLaw)Whatkindofforcesarethere?IdentifyingforcesWhatdoforcesdo?Newton’sSecondLaw

PHYS101

Section3.7ProjectileMotion:SolvingProblems

©2015PearsonEducation,Inc.

ProjectileMotionProblems


Text:p.83

TheRangeofaProjectileTherange ofaprojectileisthehorizontaldistancetraveled.

Forsmallerobjectsairresistanceiscritical,andthemaximumrangecomesatananglelessthan45°.


QuickCheck3.18Projectiles1and2arelaunchedoverlevelgroundwiththesamespeedbutatdifferentangles.Whichhitsthegroundfirst?Ignoreairresistance.

◦ Projectile1hitsfirst.◦ Projectile2hitsfirst.◦ Theyhitatthesametime.◦ There’snotenoughinformationtotell.


ExampleProblemAgrasshoppercanjumpadistanceof30in(0.76m)fromastandingstart.

◦ Ifthegrasshoppertakesoffattheoptimalangleformaximumdistanceofthejump,whatistheinitialspeedofthejump?

◦ Mostanimalsjumpataloweranglethan45°.Supposethegrasshoppertakesoffat30° fromthehorizontal.Whatjumpspeedisnecessarytoreachthenoteddistance?


BrainFart𝑥" = 0

𝑥% = 𝐷

(𝑣))" = 𝑣 cos 𝜃

(𝑣))% = 𝑣 cos 𝜃

𝑎) = 0

𝑦" = 0

𝑦% = 𝐷

(𝑣1)" = 𝑣 sin 𝜃

(𝑣1)% = −𝑣 sin 𝜃

𝑎1 = −𝑔

PHYS101

𝑥% = 𝑥" + (𝑣))"∆𝑡 +:; 𝑎)∆𝑡

;

𝐷 = 𝑣 cos 𝜃 ∆𝑡

∆𝑡 = <= >?@ A

(𝑣1)%= (𝑣1)" + 𝑎1∆𝑡

−𝑣 sin 𝜃 = 𝑣 sin 𝜃 − 𝑔∆𝑡

2𝑣 sin 𝜃 = 𝑔∆𝑡 = g <= >?@ A

𝒗𝟐 = 𝒈𝑫𝟐 𝐬𝐢𝐧 𝜽 𝐜𝐨𝐬 𝜽 or𝑫 = 𝟐 𝒗

𝟐

𝒈 𝒔𝒊𝒏𝜽 𝒄𝒐𝒔𝜽

Section4.1MotionandForces


WhatCausesMotion?

Intheabsenceoffriction,ifthesledismoving,itwillstayinmotion.


WhatCausesMotion?


WhatIsaForce?

Aforce isapush orapull.

Aforceactsonanobject.

Everyforcehasanagent,somethingthatactsorpushesorpulls.


WhatIsaForce?Aforce isavector.Thegeneralsymbolforaforceisthevectorsymbol.ThesizeorstrengthofsuchaforceisitsmagnitudeF.

Contactforcesareforcesthatactonanobjectbytouchingitatapointofcontact.

Long-rangeforces areforcesthatactonanobjectwithoutphysicalcontact.


Fr

ForceVectors


Text:p.100

ForceVectors


CombiningForcesExperimentsshowthatwhenseveralforcesareexertedonanobject,thecombinetoformanetforcethatisthevectorsumofalltheforces:

Thenetforceissometimescalledtheresultantforce.Itisnotanewforce.Instead,weshouldthinkoftheoriginalforcesbeingreplacedby.


1 2 3, , ,F F Fr r r

K

netFr

COMP:Symbolswithoverarrows areMathType

QuickCheck4.1Thenetforceonanobjectpointstotheleft.Twoofthreeforcesareshown.Whichisthemissingthirdforce?

A. B. C. D.


QuickCheck4.1Thenetforceonanobjectpointstotheleft.Twoofthreeforcesareshown.Whichisthemissingthirdforce?

Vertical components cancelA. B. C. D.


Section4.2AShortCatalogofForces


WeightThegravitationalpulloftheearthonanobjectonornearthesurfaceoftheearthiscalledweight.

Theagentfortheweightforcesistheentireearthpullingonanobject.

Anobject’sweightvectoralwayspointsverticallydownward, nomatterhowtheobjectismoving.


SpringForce

Springscomeininmanyforms.

Whendeflected,theypushorpullwithaspringforce.


TensionForce

Whenastringorropeorwirepullsonanobject,itexertsacontactforcethatwecallthetensionforce.

Thedirectionofthetensionforceisalwaysinthedirectionofthestringorrope.


NormalForceTheforceexertedonanobjectthatispressingagainstasurfaceisinadirectionperpendicular tothesurface.

Thenormalforce istheforceexertedbyasurface(theagent)againstanobjectthatispressingagainstthesurface.


NormalForceThenormalforceisresponsibleforthe“solidness”ofsolids.

Thesymbolforthenormalforceis.


nr

COMP:n[overarrow]isMathType.

FrictionFriction,likethenormalforce,isexertedbyasurface.

Thefrictionalforceisalwaysparalleltothesurface.

Kineticfriction,denotedby,actsasanobjectslidesacrossasurface.Kineticfrictionisaforcethatalways“opposesthemotion.”

Staticfriction,denotedby,istheforcethatkeepsanobject“stuck”onasurfaceandpreventsitsmotionrelativetothesurface.Staticfrictionpointsinthedirectionnecessarytopreventmotion.


kfr

sfr

Friction


DragTheforceofafluid(likeairorwater)onamovingobjectiscalleddrag.

Likekineticfriction,dragpointsoppositethedirectionofmotion.

Youcanneglectairresistanceinallproblemsunlessaproblemexplicitlyasksyoutoincludeit.


ThrustThrust isaforcethatoccurswhenajetorrocketengineexpelsgasmoleculesathighspeed.Thrustisaforceoppositethedirectioninwhichtheexhaustgasisexpelled.


ElectricandMagneticForcesElectricityandmagnetism,likegravity,exertlong-rangeforces.Theforcesofelectricityandmagnetismactonchargedparticles.Theseforces—andtheforcesinsidethenucleus—won’tbeimportantforthedynamicsproblemsweconsiderinthenextseveralchapters.


QuickCheck4.2Aballrollsdownaninclineandoffahorizontalramp.Ignoringairresistance,whatforceorforcesactontheballasitmovesthroughtheairjustafterleavingthehorizontalramp?

◦ Theweightoftheballactingverticallydown.◦ Ahorizontalforcethatmaintainsthemotion.◦ Aforcewhosedirectionchangesasthedirectionofmotionchanges.◦ Theweightoftheballandahorizontalforce.◦ Theweightoftheballandaforceinthedirectionofmotion.


QuickCheck 4.2Aballrollsdownaninclineandoffahorizontalramp.Ignoringairresistance,whatforceorforcesactontheballasitmovesthroughtheairjustafterleavingthehorizontalramp?

◦ Theweightoftheballactingverticallydown.◦ Ahorizontalforcethatmaintainsthemotion.◦ Aforcewhosedirectionchangesasthedirectionofmotionchanges.◦ Theweightoftheballandahorizontalforce.◦ Theweightoftheballandaforceinthedirectionofmotion.


QuickCheck4.3Asteelbeamhangsfromacableasacraneliftsthebeam.Whatforcesactonthebeam?

◦ Gravity◦ Gravityandtensioninthecable◦ Gravityandaforceofmotion◦ Gravityandtensionandaforceofmotion


QuickCheck4.4Abobsledderpusheshersledacrosshorizontalsnowtogetitgoing,thenjumpsin.Aftershejumpsin,thesledgraduallyslowstoahalt.Whatforcesactonthesledjustaftershe’sjumpedin?

◦ Gravityandkineticfriction◦ Gravityandanormalforce◦ Gravityandtheforceofthepush◦ Gravity,anormalforce,andkineticfriction◦ Gravity,anormalforce,kineticfriction,andtheforceofthepush


Section4.3IdentifyingForces


IdentifyingForces


Text:p.105

IdentifyingForces


ConceptualExample4.1:IdentifyingforcesonabungeejumperAbungeejumperhasleaptoffabridgeandisnearingthebottomofherfall.Whatforcesarebeingexertedonthebungeejumper?


ConceptualExample4.2:IdentifyingforcesonaskierAskierisbeingtowedupasnow-coveredhillbyatowrope.Whatforcesarebeingexertedontheskier?


Section4.4WhatDoForcesDo?


WhatDoForcesDo?Howdoesanobjectmovewhenaforceisexertedonit?


WhatDoForcesDo?

Astheblockstartstomove,inordertokeepthepullingforceconstantyoumustmoveyourhandinjusttherightwaytokeepthelengthoftherubberband—andthustheforce—constant.


WhatDoForcesDo?Theexperimentalfindingsofthemotionofobjectsactedonbyconstantforcesare:

Anobjectpulledwithaconstantforcemoveswithaconstantacceleration.

Accelerationisdirectlyproportionaltoforce.

Accelerationisinverselyproportional toanobject’smass.


QuickCheck4.5Acartispulledtotherightwithaconstant,steadyforce.Howwillitsaccelerationgraphlook?


A. B. C.

QuickCheck4.5Acartispulledtotherightwithaconstant,steadyforce.Howwillitsaccelerationgraphlook?


A constant force produces a constant acceleration.A. B. C.

Example4.4FindingthemassofanunknownblockWhenarubberbandisstretchedtopullona1.0kgblockwithaconstantforce,theaccelerationoftheblockismeasuredtobe3.0m/s2.

Whenablockwithanunknownmassispulledwiththesamerubberband,usingthesameforce,itsaccelerationis5.0m/s2.

Whatisthemassoftheunknownblock?

PREPARE Eachblock’saccelerationisinverselyproportionaltoitsmass.


Example4.4Findingthemassofanunknownblock(cont.)SOLVE WecanusetheresultoftheInverselyProportionalRelationshipsboxtowrite

ASSESS Withthesameforceapplied,theunknownblockhadalargeraccelerationthanthe1.0kgblock.Itmakessense,then,thatitsmass—itsresistancetoacceleration—islessthan1.0kg.


Section4.5Newton’sSecondLaw


Newton’sSecondLawAforcecausesanobjecttoaccelerate.

Theaccelerationa isdirectlyproportionaltotheforceFandinverselyproportionaltothemassm:

Thedirectionoftheaccelerationisthesameasthedirectionoftheforce:


Newton’sSecondLaw


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