phy101: major concepts in physics i

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PHY101:MajorConceptsinPhysicsI

Photo:J.M.Schwarz

CourseWebpage:jmschwarztheorygroup.org/phy101/Thefirstthirdofthecoursescheduleispostedattheendofthesyllabus.Ourfirstin-classexamisscheduledforWednesday,October6.Wewillholdanin-classreviewsessionnextMonday,October4.

Lab:Lab3finishestoday.Lab4startstomorrow.Pleaselookoverthelabforabout10minutesbeforegoingtoyourlabsection.

Homework:HW4 isduethisFriday,October1, by3PMinyourTA’smailboxjustoutsideofPB201.

JoinourclassSlackpage:https://join.slack.com/t/phy101fall202-6yo4297/shared_invite/zt-vfuqm8l3-GxNF4Mj5uBe6c9WxmebNjw

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Thirdsetofthreebigquestions

•Whatareforces?

• Whatlawsgovernforces?

• Howcanweapplythelawstosolveproblems?

4

Tounderstandhowmotionisinduced,defineasystem.Forcescaninducemotion.

Simplyput,aforce isa“push”or“pull”onanobjectinasystem.

Forcesarevectors!

Wecandescribethisforceviaaforcediagram!

Newton’sFirstLawNewton’sfirstlawofmotionForanobserverinaninertialreferenceframe,whennootherobjectsexertforcesonasystemorwhentheforcesexertedonthesystemaddtozero,thesystemcontinuesmovingatconstantvelocity(includingremainingatrest).

Newton’sSecondLaw

Newton’ssecondlawTheacceleration ofasystemisproportionaltothevectorsumofallforcesbeingexertedonthesystemandinverselyproportionaltothemassmofthesystem:

on S 1 on S 2 on SS

S S

(3.6)F F Fam m

+ += =å

r r rr L

Herethesubscripts1and2standfortheobjectsexertingforcesonthesystem.Theaccelerationofthesystempointsinthesamedirectionasthevectorsumoftheforces.

⇀⇀ ……

⇀⇀⇀⇀

Onelastforcelaw….

ForcesComeinPairs:Newton’sThirdLaw

Whentwoobjectsinteract,object1exertsaforceonobject2.Object2inturnexertsanequal-magnitude,oppositelydirectedforceonobject1:

1 on 2 2 on 1 3( ).9F F= -r r

Notethattheseforcesareexertedondifferentobjectsandcannotbeaddedtofindthesumoftheforcesexertedononeobject.

⇀ ⇀

Alargetruckcollideshead-onwithasmallcompactcar.Duringthecollision:

A.Thetruckexertsagreateramountofforceonthecarthanthetruckexertsonthecar.B.Thecarexertsagreateramountofforceonthetruckthanthetruckexertsonthecar.C.Thetruckexertsaforceonthecarbutthecardoesnotexertaforceonthetruck.D.Thetruckexertsthesameamountofforceonthecarasthecarexertsonthetruck.

QuickPollQuestion1

https://forms.gle/4j5tFXmzEe5UVC8q6

Let’sdoanotherexampleproblem

Example3.7:Hairdryerscontainasmallpropellerthatpushesairawayfromthedryerthroughanozzle.Youplaceahairdryeronascalewiththenozzlepointingup,anditreads4.40N.Whenyouturnthehairdryeron,sothatthehairdryerispushingtheairupward,thereadingofthescaleincreasesto4.85N.Explainthechangethescalereading.

Abookremainsatrestonatable.Whichofthefollowingistheforcepaired,accordingtoNewton'sThirdLaw,totheforcethatEarthexertsonthebook?A)Theforcethatthebookexertsonthetable.B)Theforcethatthetableexertsonthebook.C)TheforcethatthebookexertsonEarth.D)TheforcethattheEarthexertsonthetable.E)TheforcethatthetableexertsontheEarth.

QuickPollQuestion2

https://forms.gle/CsojSyxw156sb22b9

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Chapter3Textbookquestions

•Howdoseatbeltsandairbagssavelives?

• Ifyoustandonabathroomscaleinamovingelevator,doesitsreadingchange?

• Canaparachutistsurviveafalliftheparachutedoesnotopen?

TwoNewBig Questions(Chapter4)

• Howdoesonedescribemotionintheplanewithconstantacceleration?

• HowdoesoneapplyNewton’slawsintheplane?

Chapter4:ApplicationofNewton’sLaws

• Howdoesknowingphysicshelphumancannonballssafelyperformtheirtricks?

• Whowouldwinaracedownawaterslide:anadultorasmallchild?

• Howdoesfrictionhelpuswalk?

OperationswithVectors• Thelengthofavectorarrowisthevector’smagnitude.

• Theorientationofeachvectorisdeterminedbythedirectionofthearrow.

• Avectorhasatail(thepointwhereitoriginates)andahead(thetipofthearrow).

• Theminussignusedinfrontofavectormeansthevectorhasthesamemagnitudeasapositivevectorbutpointsintheoppositedirection.

OperationswithVectorsAddingvectors• Avectorcanbemovedfromonelocationtoanotheraslongasthereisnochangeinmagnitudeordirection.

• Tofindresultantofaddingtwovectors,thetailofonevectorisplacedattheheadofanothervector.

• Tofindresultantofaddinganynumberofvectors,thesameprocedureof“tailtohead”isusedforallofthem.

OperationswithVectorsAddingvectors

Tailtoheadmethod

OperationswithVectorsSubtractingvectors

Tailtoheadmethod

Findingvectorcomponents

• Wecanreplaceanyforcewithtwoperpendicularforcesaslongastheperpendicularforcesgraphicallyaddtotheoriginalforce.

• Theperpendicularforcesarealongthex- andy-axisandarecalledthex- andy-vectorcomponentsoftheoriginalforce.

Vectorcomponentstoscalarcomponents

• Theadvantagewithscalarcomponentsisthattheyarenumberswithsigns,whichcanbeaddedandsubtractedmoreeasilythanvectorquantities.

Findingscalarcomponents

Findingscalarcomponents

offorce,acceleration,velocity

ProjectileMotion

Projectilesareobjectslaunchedatananglerelativetoahorizontalsurface.WecanuseNewton’ssecondlawtoanalyzeandexplaintheirprojectilemotion.

ProjectileMotion

ProjectileMotion

Areverticalaspectsofprojectilemotionindependentfromhorizontalaspects?

ProjectileMotion


A. NoB. Yes

https://forms.gle/HUgYgR9tncbJA1pXA

ProjectileMotion


Considerthefollowingtestingexperiment.

ProjectileMotion

QuantitativeProjectileMotion

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PMExampleProblem:Anobjectisprojectedfromtheorigin.Theinitialvelocitycomponentsarev0x =7.07m/s,andv0y =7.07m/s.Determinethexandypositionoftheobjectat0.2secondintervalsfor1.4seconds.Alsoplottheresults.

tvx

tatvy

ix

yiy

D=D

D+D=D 2

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Since the object starts from the origin, Dy and Dx will represent the location of the object at time Dt.

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t (sec) x (meters) y (meters)0 0 0

0.2 1.41 1.220.4 2.83 2.040.6 4.24 2.480.8 5.66 2.521.0 7.07 2.171.2 8.48 1.431.4 9.89 0.29

PMExampleProblemcontinued:

35

0

2

4

6

8

10

12

0 0.5 1 1.5

t (sec)

x,y

(m)

Thisisaplotofthexposition(bluepoints)andyposition(pinkishpoints)oftheobjectasafunctionoftime.


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0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10

x (m)

y (m

)

Thisisaplotoftheypositionversusxpositionfortheobject(itstrajectory).

Theobject’spathisaparabola.

AnotherPMExampleProblemStephanieSmithHavensisshotfromacannonatspeed20m/s.Theendofthecannon’sbarrelis8.0mabovetheground,andthebarrelisorientedat53degreesabovethehorizontal.Stephanieenjoysherflightuntilshetouchesthenet,positioned2.0mabovegroundlevel.Thenetstretchestoreducetheforceexertedonherwhilestoppingher.

a) DeterminethetimeStephaniespendsinflight.b) Determinethehorizontaldistancefromtheendofthebarrel

wherethenetshouldbeplacedsothatshelandssafely.

ProjectileMotion

Whatisthe timeofflightandtherange?

ProjectileMotion

Consideranothertestingexperiment.

Amovingcartthatverticallyejectsaball.

phy101: major concepts in physics i

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