《As物理历年真题分类汇编》

Chapter3Forces

SectionAMultipleChoice

1Acylindricalblockofwoodhasacross-sectionalareaAandweight

W,Itistotallyimmersedinwaterwithitsaxisvertical.Theblock

experiencespressuresptand△atitstopandbottomsurfaces

respectively.

Whichofthefollowingexpressionsisequaltotheupthrustonthe

block?

02s)

AzX

(A夕)

\-/+W

/\

B夕

\lA-7

c/夕\

X(A-/)

D/夕\

\(A-z)-w

2Thevectordiagramshowsthreecoplanarforcesactingonanobjectat

P.

02s)

Themagnitudeoftheresultantofthesethreeforcesis1N.

Whatisthedirectionofthisresultant?

A(BXC/D/

3Asubmarinedescendsverticallyatconstantvelocity.Thethreeforces

actingonthesubmarineareviscousdrag,upthrustandweight.

(02s)

Whichrelationshipbetweentheirmagnitudesiscorrect?

Aweight<drag

Bweight=drag

Cweight<upthrust

Dweight>upthrust

4Aruleroflength0.30mispivotedatitscentre.Equalandopposite

forcesofmagnitude2.ONareappliedtotheendsoftheruler,creating

acoupleasshown.

2.0N

50°

ruler—/

/K.

//pivot

2.0N

Whatisthemagnitudeofthetorqueofthecoupleontherulerwhen

itisinthepositionshown?

(02s)

A0.23NmB0.39NmC0.46NmD0.60Nm

5ThediagramshowstwovectorsXandY.

(02s)

InwhichvectortriangledoesthevectorZshowthemagnitudeand

directionofvectorX—Y?

D

6uniformmetreruleofmass100gissupportedbyaknife-edgeatthe

40cmmarkandastringatthe100cmmark.Thestringpassesround

africtionlesspulleyandcarriesamassof20gasshowninthe

diagram.(02w)

020406080100

A

20g.

Atwhichmarkontherulemusta50gmassbesuspendedsothatthe

rulebalances?

A4cmB36cmC44cmD96cm

7Thediagramsrepresentsystemsofcoplanarforcesactingatapoint.

Thelengthsoftheforcevectorsrepresentthemagnitudesoftheforces.

(02w)

Whichsystemofforcesisinequilibrium?

ABD

8Whatismeantbytheweightofanobject?

(02w)

Athegravitationalfieldactingontheobject

Bthegravitationalforceactingontheobject

Cthemassoftheobjectmultipliedbygravity

Dtheobject'smassmultipliedbyitsacceleration

9Whichofthefollowingpairsofforces,actingonacircularobject,

constitutesacouple?

(02w)

2F-

F

F

10ApendulumbobisheldstationarybyahorizontalforceH.Thethree

forcesactingonthebobareshowninthediagram.

(02w)

ThetensioninthestringofthependulumisT.Theweightofthe

pendulumbobisW.

Whichstatementiscorrect?

A〃=Tcos30BT=Hsin30CW二Tcos30DF=7sin30

11Twoforces,eachof10N,actatapointPasshowninthediagram.

Theanglebetweenthedirectionsoftheforcesis120°.

(03s)

A5NBIONC17ND20N

12Thediagramshowsfourforcesappliedtoacircularobject.

(03s)

Whichofthefollowingdescribestheresultantforceandresultant

torqueontheobject?

resultantforceresultanttorque

Azerozero

Bzeronon-zero

Cnon-zerozero

Dnon-zeronon-zero

13Aballoonisacteduponbythreeforces,weight,upthrustandsideways

forceduetothewind,asshowninthediagram.

(03s)

upthrust

Whatistheverticalcomponentoftheresultantforceontheballoon?

A500NB1000NC10000ND10500N

14Aforceof5Nmayberepresentedbytwoperpendicularcomponents0Y

andOXasshowninthediagram,whichisnotdrawntoscale.

(03w)

0Yisofmagnitude3N.

WhatisthemagnitudeofOX?

A2NB3NC4ND5N

15Ahingeddoorisheldclosedinthehorizontalpositionbyacable.

Threeforcesactonthedoor:theweightWofthedoor,thetension

Tinthecable,andtheforceHatthehinge.

(03w)

Whichlistgivesthethreeforcesinincreasingorderofmagnitude?

A〃7；/BC叫〃TD叫7；〃

16Aspannerisusedtotightenanutasshown.

(03w)

AforceFisappliedatright-anglestothespanneratadistanceof

0.25mfromthecentreofthenut.Whenthenutisfullytightened,

theappliedforceis200N.

Whatistheresistivetorque,inananticlockwisedirection,

preventingfurthertightening?

A8NmB25NmC50NmD800Nm

17Twoparallelforces,eachofmagnitudeF,actonabodyasshown.

(03w)

Whatisthemagnitudeofthetorqueonthebodyproducedbythese

forces?

AFdBFsC2FdD2Fs

18Thediagramshowsasignofweight20Nsuspendedfromapole,attached

toawall.Thepoleiskeptinequilibriumbyawireattachedatpoint

Xofthepole.

(04s)

TheforceexertedbythepoleatpointXisF,andthetensioninthe

wireis40N.

WhichdiagramrepresentsthethreeforcesactingatpointX?

19Auniformbeamofweight50Nis3.0mlongandissupportedonapivot

situated1.0mfromoneend.WhenaloadofweightWishungfromthat

end,thebeamisinequilibrium,asshowninthediagram.

(04s)

3.0m

1.0m

WhatisthevalueofW?

A25NB50NC75ND100N

20Whichtwovectordiagramsrepresentforcesinequilibrium?

(04w)

APandQ

BQandR

CRandS

DSandP

21Alonguniformbeamispivotedatoneend.Aforceof300Nisapplied

toholdthebeamhorizontally.

(04w)

300N

ffhatistheweightofthebeam?

A300NB480NC600ND960N

22Thevectordiagramshowsthreecoplanarforcesactingonanobject

a

t

P

0

5

s

Themagnitudeoftheresultantofthesethreeforcesis1N.

Whatisthedirectionofthisresultant?

AlB\C/DZ

23AnL-shapedrigidleverarmispivotedatpointP.

(05s)

Threeforcesactontheleverarm,asshowninthediagram.

Whatisthemagnitudeoftheresultantmomentoftheseforcesabout

pointP?

A30NmB35NmC50NmD90Nm

24Whatisthecentreofgravityofanobject?

(05s)

Athegeometricalcentreoftheobject

Bthepointaboutwhichthetotaltorqueiszero

Cthepointatwhichtheweightoftheobjectmaybeconsideredto

act

Dthepointthroughwhichgravityacts

25Astoneisprojectedhorizontallyinavacuumandmovesalongapath

asshown.Xisapointonthispath.XVandXHareverticaland

horizontallinesrespectivelythroughX.XTisthetangenttothepath

atX.(05w)

AlongwhichdirectionordirectionsdoforcesactonthestoneatX?

AXVBXHCXVandXHDXT

26Auniformbeamofweight100NispivotedatPasshown.Weightsof

10Nand20Nareattachedtoitsends.

(05w)

Thelengthofthebeamismarkedoffat0.1mintervals.

Atwhichpointshouldafurtherweightof20Nbeattachedtoachieve

equilibrium?

0.6m0.4m

♦>V----------------------------------------------►

0.1m

V_►ABCD

10N20N

27Thediagramshowsfourforcesappliedtoacircularobject.

(05w)

30N

30N

Whichofthefollowingdescribestheresultantforceandresultant

torqueontheobject?

resultantforceresultanttorque

Anon-zeronon-zero

Bnon-zerozero

Czeronon-zero

Dzerozero

28ThegravitationalfieldstrengthonthesurfaceofplanetPisone

tenthofthatonthesurfaceofplanetQ.

(05w)

OnthesurfaceofP,abodyhasitsmassmeasuredtobe1.0kgand

itsweightmeasuredtobe1.0N.

Whatresultsareobtainedformeasurementsofthemassandweightof

thesamebodyonthesurfaceofplanetQ?

massonQweightonQ

A1.Okg0.IN

B1.Okg10N

C10kg10N

D10kg100N

29AforceFisappliedtoabeamatadistancedfromapivot.Theforce

actsatanangle0toalineperpendiculartothebeam.

(06s)

Whichcombinationwillcausethelargestturningeffectaboutthe

pivot?

Fde

Alargelargelarge

Blargelargesmall

Csmallsmalllarge

Dsmalllargesmall

30Arigiduniformbaroflength2.4mispivotedhorizontallyatits

m

i

d

P

o

i

n

t

0

6

s

0.8m0.8m

A

200NJLL300N

Weightsarehungfromtwopointsofthebarasshowninthediagram.

Tomaintainhorizontalequilibrium,acoupleisappliedtothebar.

Whatisthetorqueanddirectionofthiscouple?

A40NmclockwiseB40Nmanticlockwise

C80NmclockwiseD80Nmanticlockwise

31Thediagramsshowthreeforcesactingonabody.

(06s)

Inwhichdiagramisthebodyinequilibrium?

32ArigidcirculardiscofradiusrhasitscentreatX.Anumberof

forcesofequalmagnitudeFactattheedgeofthedisc.Alltheforces

areintheplaneofthedisc.

(06w)

Whicharrangementofforcesprovidesamomentofmagnitude2Frabout

X?

AB

F

33Threecoplanarforces,eachofmagnitude10N,actthroughthesame

pointofabodyinthedirectionsshown.

(06w)

Whatisthemagnitudeoftheresultantforce?

AONB1.3NC7.3ND10N

34Whichforceiscausedbyapressuredifference?

(06w)

Afriction

Bupthrust

Cviscousforce

Dweight

35Two8.0Nforcesactateachendofabeamoflength0.60m.Theforces

areparallelandactinoppositedirections.Theanglebetweenthe

forcesandthebeamis60°.

(07s)

Whatisthetorqueofthecoupleexertedonthebeam?

A2.4NmB4.2NmC4.8NmD9.6Nm

36Whatismeantbytheweightofanobject?

(07s)

Athegravitationalfieldactingontheobject

Bthegravitationalforceactingontheobject

Cthemassoftheobjectmultipliedbygravity

Dtheobject'smassmultipliedbyitsacceleration

37Thediagramshowsaplanviewofadoorwhichrequiresamomentof

12Nmtoopenit.

(07w)

Whatistheminimumforcethatmustbeappliedatthedoor'smidpoint

toensureitopens?

A4.8NB9.6NC15ND30N

38Thesymbolrepresentstheaccelerationoffreefall.

(07w)

Whichofthesestatementsiscorrect?

Aisgravity.

Bisreducedbyairresistance.

Cistheratioweight/mass.

Distheweightofanobject.

39Whichtwovectordiagramsrepresentforcesinequilibrium?

(07w)

RS

、/

APandQBQandRCRandSDSandP

40Thediagramshowsadiving-boardheldinpositionbytworodsXand

Y.

(01s)

Whichadditionalforcesdotheserodsexertontheboardwhenadiver

ofweight600Nstandsontheright-handend?

atX(downwards)atY(upwards)

A400N1000N

B600N1200N

C900N600N

D900N1500N

41AsmallballofweightWissuspendedbyalightthread.Astrongwind

blowshorizontally,exertingaconstantforceFontheball.The

threadmakesanangle°totheverticalasshown.

(01s)

Whichequationcorrectlyrelates°,FandW?

Acos9-FIWBsin°=FIW

Ctan9-FIWDtan°=WIF

42Twoco-planarforcesactontherimofawheel.Theforcesareequal

n

m

a

g

n

i

t

u

d

e

(

0

1

s

Whicharrangementofforcesisacouple?

43Twoforcesactonacirculardiscasshown.

(01s)

44Whatisthedefinitionofforce?

(Olw)

Athemassofabodymultipliedbyitsacceleration

Bthepowerinputtoabodydividedbyitsvelocity

Ctherateofchangeofmomentumofabody

Dtheworkdoneonabodydividedbyitsdisplacement

45Astreetlampisfixedtoawallbyametalrodandacable.

(Olw)

46Awheelofradius0.70mhasacoupleappliedtoitasshown.

(Olw)

0.70m

Whatisthetorqueonthewheel?

A0B28NmC56NmD112Nm

47TwoforcesXandYactatapointPasshown.Thelengthsofthelines

representthemagnitudesoftheforces.

(Olw)

48Atrailerofweight30KNishitchedtoacabatthepointXasshown

inthediagrambelow.

(Olw)

Whatupwardforceisexertedbythecabonthetraileratthepoint

X?

A3KnB15kNC30kND60kN

49AballofweightWslidesalongasmoothhorizontalsurfaceuntilit

fallsofftheedgeattimeT.

(Olw)

WhichgraphrepresentshowtheresultantverticaldownwardsforceF,

actingontheball,varieswithtimetastheballmovesfromposition

XtopositionY?

SectionBStructuredQuestions

1(a)Statetheconditionsnecessaryfortheequilibriumofabodywhich

isacteduponbyanumberofforces.

(Olw)

1.................................................................

2

(b)ThreeidenticalspringsSbS2andS3areattachedtoapointAsuch

thattheanglebetweenanytwoofthespringis120°,asshown

inFig.3.1.

Fig.3.1

ThespringshaveextendedelasticallyandtheextensionsofSiand

S2arex.Determine,intermsofx,theextensionofS3suchthat

thesystemofspringsisinequilibrium.Explainyourworking.

extensionofS3

=.......................：3]

(c)ThelidofaboxishingedalongoneedgeE,asshowninFig.3.2.

cord

Thelidisheldopenbymeansofahorizontalcordattachedtothe

edgeFofthelid.Thecentreofgravityofthelidisatpoint

C.

OnFig.3.2draw

(i)anarrow,labelledW,torepresenttheweightofthelid,

(ii)anarrow,labelledT,torepresentthetensioninthecord

actingonthelid,

(iii)anarrow,labelledR,torepresettheforceofthehingeon

thelid.

[3]

2(a)Explainwhatismeantbythecentreofgravityofanobject.

(02s)

.....[2]

(b)Anon-uniformplankofwoodXYis2.50mlongandweighs950N.

Force-meters(springbalances)AandBareattachedtotheplank

atadistanceof0.40mfromeachend,asillustratedinFig.3.1.

Fig.3.1

Whentheplankishorizontal,force-meterArecords570N.

(i)Calculatethereadingonforce-meterB.

reading

=................N

(ii)OnFig.3.1,markalikelypositionforthecentreofgravityof

theplank.

(iii)DeterminethedistanceofthecentreofgravityfromtheendX

oftheplank.

distance

=...............m[6]

3(a)Definethemomentofaforce.

(03w)

...............[2]

(b)Statethetwoconditionsnecessryforabodytobeinequilibrium.

1..................................................................

2..................................................................

...............[2]

(c)TwoparallelatringsSiandS2areattachedtoadiscofdiameter

12cm,asshowninFig.3.1.

Thediscisfreetorotateaboutanaxisnormaltoitsplane.The

axispasses

throughthecentreCofthedisc.

Aleveroflength30cmisattachedtothedisc.WhenaforceFis

appliedatrightanglestotheleveratitsend,equalforcesare

producedinSiandS2.Thedi