Tony Ford Science Friday, 26 September 2014 Page 1
7.1LessonBalancedversusUnbalancedforces.
ReviewForces:
1.
2.
3.
4.
5.
Aforceisapushorapullcausedbydirectcontact.
Itcanalsobecausedbynoncontactforcescalled=ieldforces.
Forcescausechangeinmotion.
Forcescancausechangeinshape.
Frictionisaforcethatcanbehelpfulorahindrance.
Giveexamplesofeachoftheabovesituations
1.
2.
3.
4.
5.
NewUnderstandingsofforceinthisunit:
1. Forcesactinacertaindirectionwithacertainsize.Thisisrepresentedby
straightarrows.
2. ForcesaremeasuredinNewtons.
3. Ifthereisnoforceortheforcesarebalanced,abodywillstayinits
presentstate(movingorstopped).Thisiscalledinertia.
4. Unbalancedforcescauseobjectstoaccelerateordecelerate.
5. Forcesactinpairs(action/reaction)
MeasuringForces
Everyforcehastwoimportantmeasures.
1. Thesizeoftheforce:
ThesizeofaforceismeasuredinNewtonsorsymbolN.Tolifta1kgweight
wouldtakeabout10N(newtons)
Thesizeofaforcecanbemeasuredusingaspringordigital
balance.Theyworkbystretchingaspringorcompressinga
pressurepad.
Evenarubberbandcouldbeusedasaninstrumenttomeasure
force!
Q.Explainhowarubberbandmightbeusedtomeasure
force?Whatbeitslimitations?
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2. Thedirectionoftheforce:
Themotionofanobjectdependsonwheretheforceactsandinwhat
directione.g.itiseasiertopushanobjectdownhillratherthanuphill.
Large
uphill
force
Small
downhill
force
Thelongerthearrow,thelargertheforce.Thearrowshowsthedirection
theforceisactingandfromwhereitstarts.
Mostoften,morethanoneforceactsonanobject.
Example1:Athrowerwillhaveatleastoneotherforceactingontheballbesides
histhrow.
Q.Whatdoyouthinkitis?
Example2:Anobjectonatablehasatleasttwoforcesonit;Gravityforcedown
whichisitsweightandthetablepushingitup.Thisiscalledthereactionforce.
Q.Ifthetablewasn’ttherewhatwouldtheobjectdo?
Weight
Weight
Diagramwitharrowstoshowsizeand
directionofforce
Ifthetwoforcesareinoppositedirectionandthesamesizetheywillcanceleach
otherout.ThisiscalledBalancedForces!
Q.Whichdiagramabovearethetwoforcesbalanced?
Whenthishappenstheobjectdoesn’tmove,ifitwasstationarytostartwith.
Question.Inatugofwargamewherenooneiswinning,whatishappeningto
thetwopullingforces?Explainwithadiagram:
Answertwoforcescanceleachotherout
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Inertia
Balancedornoforcesatall,meansanobjectwillkeepmovingataconstant
speedorstopped.Thisisbecauseanobjectwantstostaydoingwhatithasjust
beendoing.
ThisiscalledINERTIA.
Questioncanyouthinkofanexampleofwhenanobjectwantstokeepmoving?
Answers
Infacteverysituationwhereanobjectismoving,itneedsaforcetostopit.Heavy
objectsarethemostobvious.Inaroadaccidentthedamageshowshowharditis
forsomethingtostop.
DemonstrationsofInertiaandforcesacting
ThrowingaBallintheAir
2s
1s
3s
KeyGravity
Force
Push
Force
4s
Theballmovestothepositionsshownevery1secondbutafteritleavesthehand
thereisonlyoneforceacting(gravity)ifyouignoreairresistance.Thepush
forceofthehandstopsonceitleavesthehand!
Q.Whythendoestheballkeepgoingupforawhileandawayfromthehandif
thepushforcehasstopped?
A.Inertia
DoGroupActivity:7.2NuclearReactorGame
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2.UnbalancedForces.
Unbalancedforcescause,changeorstopmotion.Whenoneforceisbiggerthan
another,thedifferencebetweenthoseforcesdecideshowmuchmovementwill
happen.Ifanobjectspeedsupitiscalledaccelerationifitslowsdownitscalled
deceleration.
Examplesofusingunbalancedforces:
1.Friction
Youlearntinyear4thatFrictionisacontactforce
duetotheroughnessofsurfacesthatpushesagainstmovement.Frictionnever
givesupbutitdoeschange:
1. Itincreasesagainstaforceasitisappliedtogetanobjectmoving.
2. Itreachesalimit.Aforceappliedbeyondthislimitstartsanobject
moving.ThislimitiscalledmaximumStaticfriction.
3. Itdropsonceanobjectstartstomovebutnevergivesup.Thisiscalled
Kineticfriction.Itmaybethefrictionagainstanobjectslidingorrollingif
it’sonwheels.
4. Whenaforceisnolongerappliedtoamovingobject,kineticfrictionwill
slowitdownuntilitstops.(e.g.braking)
5. Thesizeoffrictiondependsonthetypeandareaofthesurfacesincontact
andtheweightoftheload.
Questions:
Q1.Whycanwenotmovesomeobjects?
Answer:Thestaticfrictionisjusttoogreat.
Q2.Whatisnormallyhigher;slidingorrollingkineticfriction?
Answer:Slidingfrictionaslongasthewheelsorrollersdon’tgetstuck.
Q3.Whatchangeswhenbrakingquicklycomparedtobrakingslowlyorjust
coasting?
Answer:Frictionisincreasedbylockingthewheelsandchangingfromrollingto
slidingfriction.
Q4.Whyisitimportanttohavegoodtreadonyourtyres?
Answer:Frictionisanuisancewhenitmakesthingshardtomove,slowsthings
downorwearsthingsoutbutitisalsoveryimportantwhenbrakingorgettinga
gripwhentakingoff.Thisiswhytyreshavetohavetread.Thenthefrictionishigh
andthecarwon’tskid.
Q5.Wheelsreducefrictionbychanging:
a) thetypeofsurface
b) areaincontact
c) weightoftheobject
d) alloftheabove
Answer.Couldbealloftheabovebutmostlyitreducestheareaincontactc)
Do7.3InvestigatingFrictionandLoad
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3. Gravity
AsyoulearntinGrade4Gravityisanoncontact
force.Itactuallyexistsbetweenanytwoobjectsbut
becauseitisdependentonthemassoftheobject,
onearththeonlygravitywenoticeistheonethat
theearthmakes(becauseitissobig!)
Earth’sgravityisstrongestonthesurfaceoftheearth
and
actuallychangesaccordingtothemassoftheobject,itis
pulling
on.
Infacttheearthwillpullaheavyobjectharderthanalightobjectsothatthey
bothhitthegroundatthesametime.Prettysmartearth!
Anothernameoftheearth’sgravityforceiscalledweight.So
when
wetalkabouthowmuchsomethingweighs,wearereally
talkingabouthowmuchtheearthispullingonit.
Theweightofanobjectdependsonitsmassbuttheyare
notthesame.WeightisaforcemeasuredinNewtons.
Massistheamountofmatteranobjecthasmeasuredin
kilograms.
QuestionHowmuchdoyouweigh?
Answer:
Thisisatrickquestionbecauseifyoumeanwhatismymass?Itwillbesomany
kilogramse.g.100kg.
Butifyoumeanwhatismyweight?ItwillbeinNewtons.Generallyonearthto
convertyourmassintoweightmultiplyby10e.g.100kghasaweightof1000N.
Alsoweightdoesdependonhowclosetotheearthorsomeotherplanetyouare.
Astronautsinouterspacecanbeweightlessbecausetheyarenearnoplanetsor
moonsandhavealmostnogravityforceactingonthem.
Do7.4InvestigateMassversusWeight