Forces and Motion section 1

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InteractiveTextbook91ForcesandMotion
SECTION
1
NameClassDate
BEFOREYOUREAD
CHAPTER6ForcesandMotion
GravityandMotion
Afteryoureadthissection,youshouldbeabletoanswer
thesequestions:
•Howdoesgravityaffectobjects?
•Howdoesairresistanceaffectfallingobjects?
•Whatisfreefall?
•Whydoesanobjectthatisthrownhorizontallyfollow
acurvedpath?
HowDoesGravityAffectFallingObjects?
InancientGreece,agreatthinkernamedAristotle
saidthatheavyobjectsfallfasterthanlightobjects.For
almost2,000years,peoplethoughtthiswastrue.Then,in
thelate1500s,anItalianscientistnamedGalileoGalilei
provedthatheavyandlightobjectsactuallyfallatthe
samerate.
IthasbeensaidthatGalileoprovedthisbydropping
twocannonballsfromthetopofatoweratthesame
time.Thecannonballswerethesamesize,butonewas
muchheavierthantheother.Thepeoplewatchingsaw
bothcannonballshitthegroundatthesametime.
Whydon’theavyobjectsfallfasterthanlightobjects?
Gravitypullsonheavyobjectsmorethanitpullsonlight
objects.However,heavyobjectsarehardertomovethan
lightobjects.So,theextraforcefromgravityontheheavy
objectisbalancedbyhowmuchharderitistomove.
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STUDYTIP
PracticeAftereverypage,
stopreadingandthink
aboutwhatyou’veread.Try
tothinkofexamplesfrom
everydaylife.Don’tgoon
tothenextsectionuntilyou
thinkyouunderstand.
READINGCHECK
1.DescribeWhatdid
thepeoplewatchingthe
cannonballsseethattold
themthecannonballsfellat
thesamerate?
TAKEALOOK
2.PredictThegolfball
isheavierthanthetable
tennisball.Onthefigure,
drawthreecirclestoshow
wherethegolfballwillbeat
times2,3,and4.
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InteractiveTextbook92ForcesandMotion
SECTION1
NameClassDate
GravityandMotioncontinued
HowMuchAccelerationDoesGravityCause?
Becauseofgravity,allobjectsaccelerate,orspeedup,
towardEarthatarateof9.8meterspersecondpersec-
ond.Thisiswrittenas9.8m/s/sor9.8m/s2
.So,forevery
secondanobjectfalls,itsvelocity(speed)increasesby
9.8m/s.Thisisshowninthefigurebelow.
4.9m
v=0m/sdownward
v=9.8m/sdownward
v=19.6m/sdownward
v=_______________________
14.7m
24.5m
Firstsecond
Secondsecond
Thirdsecond
Afallingobjectacceleratesataconstantrate.Theobjectfalls
fasterandfarthereachsecondthanitdidthesecondbefore.
?
WhatIstheVelocityofaFallingObject?
Supposeyoudroparockfromacliff.Howfastisitgoing
whenitreachesthebottom?Ifyouhaveastopwatch,you
cancalculateitsfinalvelocity.
Ifanobjectstartsfromrestandyouknowhowlong
itfalls,youcancalculateitsfinalvelocitybyusingthis
equation:
vfinal
gt
Intheequation,vfinal
standsforfinalvelocityinmeters
persecond,gstandsfortheaccelerationduetogravity
(9.8m/s2
),andtstandsforthetimetheobjecthasbeen
falling(inseconds).
Iftherocktook4stohittheground,howfastwasit
fallingwhenithittheground?
Step1:Writetheequation.
vfinal
gt
Step2:Placevaluesintotheequation,andsolvefortheanswer.
vfinal
9.84s
m/s
s
39.2m/s
Thevelocityoftherockwas39.2m/swhenithitthe
ground.
MathFocus
3.CalculateHowfastisthe
ballmovingattheendofthe
thirdsecond?Explainyour
answer.
MathFocus
4.CalculateApennyis
droppedfromthetopofatall
stairwell.Whatisthevelocityof
thepennyafterithasfallenfor
2s?Showyourwork.

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InteractiveTextbook93ForcesandMotion
SECTION1
NameClassDate
GravityandMotioncontinued
HowCanYouCalculateHowLonganObject
WasFalling?
Supposesomeworkersarebuildingabridge.Oneof
themdropsametalboltfromthetopofthebridge.When
thebolthitstheground,itismoving49m/s.Howlong
doesittakethebolttofalltotheground?
Step1:Writetheequation.
t
vfinal
g
Step2:Placevaluesintotheequation,andsolvefortheanswer.
t49m/s
m/s9.8
5s
s
Theboltfellfor5sbeforeithittheground.
HowDoesAirResistanceAffectFallingObjects?
Trydroppingapencilandapieceofpaperfromthe
sameheight.Whathappens?Doesthissimpleexperiment
showwhatyoujustlearnedaboutfallingobjects?Now
crumplethepaperintoatightball.Dropthecrumpled
paperandthepencilfromthesameheight.
Whathappens?Theflatpaperfallsmoreslowlythanthe
crumpledpaperbecauseofairresistance.Airresistanceis
theforcethatopposesthemotionoffallingobjects.
Howmuchairresistancewillaffectanobjectdepends
onthesize,shape,andspeedoftheobject.Theflatpaper
hasmoresurfaceareathanthecrumpledsheet.This
causestheflatpapertofallmoreslowly.
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MathFocus
5.CalculateArockfalls
fromacliffandhitsthe
groundwithavelocityof
98m/s.Howlongdoesthe
rockfall?Showyourwork.
READINGCHECK
6.IdentifyWhichhasmore
airresistance,theflatpaper
orthecrumpledpaper?
TAKEALOOK
7.ExplainWhydoesthe
crumpledpaperfallfaster
thantheflatpaper?
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InteractiveTextbook94ForcesandMotion
SECTION1
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GravityandMotioncontinued
READINGCHECK
8.DescribeWhendoes
anobjectreachitsterminal
velocity?
TAKEALOOK
9.IdentifyAskydiveris
fallingatterminalvelocity.
Drawandlabelanarrow
showingthedirectionandsize
oftheforceduetogravityon
theskydiver.Drawandlabela
secondarrowshowingthe
directionandsizeoftheforce
ofairresistanceontheskydiver.
TAKEALOOK
10.PredictWhenair
resistanceactsontheapple
andthefeather,whichfalls
faster?
WhatIsTerminalVelocity?
Asthespeedofafallingbodyincreases,airresistance
alsoincreases.Theupwardforceofairresistancekeeps
increasinguntilitisequaltothedownwardforceofgrav-
ity.Atthispoint,thetotalforceontheobjectiszero,so
theobjectstopsaccelerating.
Whentheobjectstopsaccelerating,itdoesnotstop
moving.Itfallswithoutspeedinguporslowingdown.It
fallsataconstantvelocitycalledtheterminalvelocity.
Terminalvelocityisthespeedofanobjectwhenthe
forceofairresistanceequalstheforceofgravity.
Airresistancecausestheterminalvelocityofhailstones
tobebetween5m/sand40m/s.Withoutairresistance,they
couldreachthegroundatavelocityof350m/s!Airresistance
alsoslowsskydiverstoasafelandingvelocity.
WhatIsFreeFall?
Freefallisthemotionofanobjectwhengravityisthe
onlyforceactingontheobject.Thefigurebelowshowsa
featherandanapplefallinginavacuum,aplacewithoutany
air.Withoutairresistance,theyfallatthesamerate.
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Theparachuteincreasestheair
resistanceofthisskydiverand
slowshimtoasafeterminal
velocity.

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InteractiveTextbook95ForcesandMotion
SECTION1
NameClassDate
GravityandMotioncontinued
OrbitingObjectsAreinFreeFall
SatellitesandthespaceshuttleorbitEarth.Youmay
haveseenthatastronautsinsidetheshuttlefloatunless
theyarebeltedtoaseat.Theyseemweightless.Infact,
theyarenotweightless,becausetheystillhavemass.
Weightisameasureofthepullofgravityonanobject.
Gravityactsbetweenanytwoobjectsintheuniverse.
Everyobjectintheuniversepullsoneveryotherobject.
Everyobjectwithmasshasweight.
Theforceofgravitybetweentwoobjectsdependson
themassesoftheobjectsandhowfaraparttheyare.The
moremassivetheobjectsare,thegreatertheforceis.
Theclosertheobjects,thegreatertheforce.
YourweightisdeterminedmostlybythemassofEarth
becauseitissobigandsoclosetoyou.Ifyouwereto
moveawayfromEarth,youwouldweighless.However,
youwouldalwaysbeattractedtoEarthandtoother
objects,soyouwouldalwayshaveweight.
Astronautsfloatintheshuttlebecausetheshuttle
isinfreefall.That’sright—theshuttleisalways
falling.Becausetheastronautsareintheshuttle,theyare
alsofalling.Theastronautsandtheshuttlearefallingat
thesamerate.Thatiswhytheastronautsseemtofloat
insidetheshuttle.
IsaacNewtonfirstpredictedthiskindoffreefallin
thelate17thcentury.Hereasonedthatifacannonwere
placedonamountainandfired,thecannonballwould
falltoEarth.Yet,ifthecannonballwereshotwith
enoughforce,itwouldfallatthesameratethatEarth’s
surfacecurvesaway.Thecannonballwouldneverhitthe
ground,soitwouldorbitEarth.Thefigurebelowshows
this“thoughtexperiment.”
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READINGCHECK
11.ExplainWhenwillan
objecthavenoweight?
Explainyouranswer.
READINGCHECK
12.ExplainWhydon’tthe
astronautsintheorbiting
shuttlefalltothefloor?
CriticalThinking
13.InferComparedwith
cannonballb,whatdoyou
thinkcannonballcwoulddo?
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InteractiveTextbook96ForcesandMotion
SECTION1
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GravityandMotioncontinued
WhatMotionsCombinetoMakean
ObjectOrbit?
Anobjectisinorbit(isorbiting)whenitisgoing
aroundanotherobjectinspace.Whenthespaceshuttle
orbitsEarth,itismovingforward.Yet,theshuttleis
alsoinfreefall.Thefigurebelowshowshowthesetwo
motionscombinetocauseorbiting.
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WhatForceKeepsanObjectinOrbit?
Manyobjectsinspaceareorbitingotherobjects.The
moonorbitsEarth,whileEarthandtheotherplanetsorbit
thesun.Theseobjectsallfollownearlycircularpaths.An
objectthattravelsinacircleisalwayschangingdirection.
Ifalltheforcesactingonanobjectbalanceeachother
out,theobjectwillmoveinthesamedirectionatthe
samespeedforever.So,objectscannotorbitunlessthere
isanunbalancedforceactingonthem.Centripetalforce
istheforcethatkeepsanobjectmovinginacircular
path.Centripetalforcepullstheobjecttowardthecenter
ofthecircle.Thecentripetalforceofabodyorbitingin
spacecomesfromgravity.
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TAKEALOOK
14.IdentifyOnthefigure,
drawalineshowingthepath
thatthespaceshuttlewould
takeifgravitywerenotacting
onit.
READINGCHECK
15.IdentifyWhatmust
beappliedtoanobjectto
changeitsdirection?
TAKEALOOK
16.IdentifyDrawanarrow
onthefiguretoshowthe
directionthatcentripetal
forceactsonthemoon.

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InteractiveTextbook97ForcesandMotion
SECTION1
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GravityandMotioncontinued
WhatIsProjectileMotion?
Projectilemotionisthecurvedpathanobjectfol-
lowswhenitisthrownneartheEarth’ssurface.The
motionofaballthathasbeenthrownforwardisan
exampleofprojectilemotion.
Projectilemotionismadeupoftwoparts:horizontal
motionandverticalmotion.Horizontalmotionismotion
thatisparalleltotheground.Verticalmotionismotion
thatisperpendiculartotheground.Thetwomotionsdo
notaffecteachother.Instead,theycombinetoformthe
curvedpathwecallprojectilemotion.
Whenyouthrowaballforward,yourhandpushesthe
balltomakeitmoveforward.Thisforcegivestheball
itshorizontalmotion.Aftertheballleavesyourhand,no
horizontalforcesactontheball(ifweforgetairresis-
tancefornow).Sotheball’shorizontalvelocitydoesnot
changeafteritleavesyourhand.
However,gravityaffectstheverticalpartofprojectile
motion.Gravitypullstheballstraightdown.Allobjects
thatarethrownacceleratedownwardbecauseofgravity.
READINGCHECK
17.ListWhattwomotions
combinetomakeprojectile
motion?
CriticalThinking
18.InferIfyouareplaying
dartsandyouwanttohitthe
bulls-eye,whereshouldyou
aim?
TAKEALOOK
19.ListOnthefigure,fill
inthethreeblankswiththe
correctwords.
20.ApplyConceptsIfthere
werenoairresistance,how
fastwouldtheball’sdown-
wardvelocitybechanging?
Explainyouranswer.
aAftertheballleavesthepitcher’shand,theball’svelocity
isconstant.
bTheball’sverticalvelocityincreasesbecausecausesitto
acceleratedownward.
cThetwomotionscombinetoformapath.
a
b
c
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InteractiveTextbook98ForcesandMotion
NameClassDate
Section1Review
SECTIONVOCABULARY
freefallthemotionofabodywhenonlythe
forceofgravityisactingonthebody
projectilemotionthecurvedpaththatan
objectfollowswhenthrown,launched,or
otherwiseprojectednearthesurfaceofEarth
terminalvelocitytheconstantvelocityofa
fallingobjectwhentheforceofairresistance
isequalinmagnitudeandoppositein
directiontotheforceofgravity
1.ExplainIsaparachutistinfreefall?Whyorwhynot?
2.IdentifyCauseandEffectCompletethetablebelowtoshowhowforcesaffect
objects.
CauseEffect
Gravityactsonafallingobject.
Thefallingobjectreachesterminalvelocity.
3.CalculateArockatrestfallsoffacliffandhitsthegroundafter3.5s.Whatisthe
rock’svelocityjustbeforeithitstheground?Showyourwork.
4.IdentifyWhatforcemustbeappliedtoanobjecttokeepitmovingina
circularpath?
5.ExplainWhichpartofprojectilemotionisaffectedbygravity?Explainhow
itisaffected.