Hướng dẫn giải chi tiết đề thi tuyển sinh ĐHCĐ môn vật lý (NXPowerLite copy)

530.076

PHAN VAN HUAN

H561D

HlidNG DAN GIAI CHI TIET
DE THI TUYEN SINK
DAI HOC. CAO B&NC

VAT LI

H Nha xuat ban Dai hoc Quoc gia Ha NOi


bJO. 01±__

'-W5t>/< 0

PHANVANHUAN

DE THI DAI HQC - CAO DANG

M6n

VAT LI

NHA XUAT BAN DAI HQC QUOC GIA HA NQI


N H A X U A T B A N DAI H Q C Q U O C G I A H A NOI

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In xong va nop luu chieu quy I nam 2013.


DE THI TUYEN SINH DAI HQC, CAO DANG NAM 2008
MonthI: V A T L I - K h o i A
Thdi gian lam bai: 90 phut
Cdu 1: K h i chieu I a n liftft h a i buTc xa c6 t a n so l a f^, iz ( v d i fi < £ 3 ) vko .
mot qua cau k i m loai dSt c6 lap t h i deu xay r a h i | n ttfcfng quang d i e n
v d i dien the cifc d a i cua qua cau I a n lufdt la V i , V2. Neu chieu dong t h d i
hai bufc xa t r e n v^o qua cau nay t h i d i e n the cifc d a i cua no \k:
A.

V2.

B.

(Vi + V2).

C.

Vi.

D, IVi -

V2I.

Giai
Vi

fi < f2 n e n V2 > Vi.

Neu chieu dong t h d i h a i bufc xa t r e n v^o qua cau t h i d i $ n the cifc
dai cua no chi hang d i e n the I d n hcfn, do c h i n h la V2.
Vdy chon dap an A.
Cdu 2: T r o n g t h i n g h i e m ve song dCrn^, t r e n m o t stfi day d a n h 6 i d^i
1,2 m v d i h a i dau co d i n h , ngUdi t a quan sat t h a y ngoai h a i dau day co
d i n h con co h a i d i e m khac t r e n day k h o n g dao dong. B i e t k h o a n g t h d i
gian giOfa h a i I a n l i e n tiep sgti day duoi t h i n g l a 0,05 s. V a n toe t r u y e n
song t r e n day l a :
A. 16 m/s.

B. 4 m/s.

C. 12 m/s.

D . g m/s.

Giai
Theo de b a i t h i t r e n day tao t h a n h 3 m u i song, m o i m i i i s6ng co
chieu dai —.
2
T a c o : 3 - = 1,2 = > = 0,8m.
2
Khoang t h d i gian giffa h a i I a n l i e n t i e p stfi day a j 6 j t h i n g l a 0,05s

nen t a co chu k i T = 0,05.2 = 0,1s.
V a n toe t r u y e n song t r e n day: ^

5i,
= ^

0 8
~ "QT ~ ^C™'^)'

Vdy chon dap an D.
Cdu 3: Doi v d i SLT Ian t r u y e n song dien tCf t h i
A. vector ciTdng do dien triJdng E va vectcf cam ufng tCr B

luon

vuong goc v d i phtTcfng t r u y e n song.
Giai Chi tiSt (3e thi TS DH, CO M6n vat li

3


B. vectcf CLfcrng do d i e n triTcfng E va vecto cam ufng tiT B luon cung
phiiong vdfi phiTctog t r u y e n song.
C. vector cam ufng tif B cung phLfcrng vdri phirorng t r u y e n song con
vector CLTcfng do dien t r i / d n g E vuong goc v d i vector cam ufng ttf B .
D. vectcf cifcfng do dien tn/cfng E cung phtfcfng v6i phiromg truyen song
c6n vector cam ufng tCf B vuong goc vdri vector ci/orng do dien trUdng E .

Giai
D o i v d i suf Ian t r u y e n song dien tCf t h i vecto cifdng do dien triicfng

E va vector cam ufng tCr B luon vuong goc vdri phiTorng t r u y e n song.
Vay chgn dap

an A.

Cdu 4: Cof n a n g cua m o t v a t dao dong dieu hoa:
A. t a n g gap doi k h i bi§n do dao dong cua v a t t a n g gap doi.
B. b i e n t h i e n t u a n hoan theo t h d i gian v 6 i chu k y b k n g chu k y dao
dong cua v a t .
C. b i e n t h i e n t u a n hoan theo thofi gian v d i chu k y bSng mot nufa
chu k y dao dong cua vat.
D . b a n g dong nSng cua v a t k h i v a t id'i v i t r i can bang.

GiSi
Cor n a n g cua m o t vat dao dong dieu hoa bang dong nang cua vat
k h i v a t tdri v i t r i can bang.
Vay chgn dap

an D.

Cdu 5: H a t n h a n '°Be c6 k h o i liTcrng 10,0135u. K h o i lifgng cua ncftron
( n a t r o n ) m n = l , 0 0 8 7 u , k h o i Itrcrng cua p r o t o n ( p r o t o n ) mp = l , 0 0 7 3 u ,
l u = 931 MeV/c^. N a n g liTcfng l i e n k e t r i e n g cua h a t n h a n '"Be
D . 632,1531 MeV.

C. 6,3215 M e V .

B. 63,2152 MeV.

A . 0,6321 M e V .

la:

Giai
N a n g Itfdng l i e n k e t r i e n g cua h a t n h a n '°Be l a :

«'

AC
(6m„+4m -meJ.QBl
AJb^V
^
= 6,3215MeV.
A
Vay chgn dap

A
an C.

Cdu 6: P h a t bieu nao sau day la s a i k h i n o i ve nang liidng dao dong
dien tii t i i (Jo (dao dong rieng) t r o n g mach dao dong dien tCr LC k h o n g
CO dien trcf thuan?
4

Giai Chi tiet de thi TS DH, CD M6n Vat li


A. tl^.ng luang tCr trifcfng cure dai b k n g n a n g Itfong d i e n tCr cua mach
dao dong.
B. N a n g Itfcfng dien trtf&ng va n a n g Itftfng tCr t n f o n g b i e n t h i e n dieu

hoa vdfi t a n so bSng mot nijfa t a n so cua ctfcfng do dong dien t r o n g mach.
C. K h i nSng lifcfng dien trtfcrng g i a m t h i n a n g liitfng tCr trifcrng t a n g .
D. Nang Itfofng dien tii cua mach dao dong bkng tong nang liigfng dien
tnfcfng tap trung cf t u dien va nang li/cfng tCr tnicfng tap trung cf cugn cam.

Giai
V i n a n g lifcfng dien triicfng va nSng Itfofng tCf triXdfng b i e n t h i e n dieu
hoa v6i chu k i bang mot nufa chu k i cifcfng do dong dien t r o n g mach nen
tan so cua no p h a i gap doi t a n so cua cifdfng do dong dien t r o n g mach.
Vdy chqn dap an B.
Cdu 7: T i a Rcfnghen c6
A. cung b a n chat v 6 i song v6 tuyen.
B. cung ban chat vdfi song a m .
C. dien t i c h a m .
D. btfdfc song Idfn hcfn biTdfc song cua t i a h o n g ngoai.

Giai
T i a Rcfnghen la bufc xa dien tii n e n k h o n g m a n g d i e n , c6 cung b a n
chat vdfi song v6 tuyen va c6 bildfc song nho hofn biidfc song cua t i a
hong ngoai.
Vdy chon dap an A.
Cdu 8: H a t n h a n A dang dufng yen t h i p h a n r a t h a n h h a t n h a n B c6
k h o i lifong ms va h a t a c6 k h o i lucfng m„. T i so giOfa dong n a n g cua
h a t n h a n B va dong n a n g cua h a t a ngay sau p h a n r a b k n g : .
A. ^ .

c.

B.

m„

D.

^

Giai
Ap dung d i n h l u a t bao toan dong liJcfng cho h a t dang dufng y e n , t a
c6:

mBVB

= m„v„.

T i so giufa dong n&ng cua h a t n h a n B va dong nSng cua h a t a
ngay sau p h a n r a l a : ^
Vdy chqn dap an D.
Giai Chi tiet de thi T S B H , C D M6n Vat II

=

-


Cdu 9: Cho doan mach di$n xoay chieu gom cuon day c6 dien trd
thuan R , m^c nol tiep v6i tu*dien. Biet hieu dien the giffa hai dau cuon
day lech pha — so v6i hi|u dien the giuTa hai dau doan mach. Moi lien
h$ giiJa dien tror thuan R vdri cam khang Z L cua cuon day

dung

khdng Zc cua tu dien la:

'

D . R=^ = ZC(ZL -

C. R ' = Zc(Zc - ZL). '

E. R2 = ZL(ZC - Z L ) .

A. R ' = ZL(ZL - Zc).

Zc).

2
Do lech pha giffa hai dau cuon day: tancp, = —^.
R

Z
Do l$ch pha giffa hai dau doan mach: tan(p = —

-Z
-.

R

Theo gia thiet 9, +(p = — nen ta c6: cot(p, =tan(p

=>^=^^-^R^=z,(z,-zJ.

Vdy chqn dap an B.
Cdu 10: Hat nhan ^^X ph6ng xa

bien thanh mot hat nhan ^^^Y

ben. Coi khoi lifcfng cua hat nhan X , Y bang so khoi cua chung tinh
theo dcfn v i u. Biet chat phong xa

X c6 chu k i bdn ra 1^ T . Ban dau

c6 mot khoi lifcfng chat ^^X, sau 2 chu Id ban ra t h i t i so giffa khoi
Iffdng cua chat Y
A. 4 - ^ .
Aj

khoi lUcfng cua chat X la:
B. 3 ^ .
A,

C. 4 ^ .
A,

Sau 2 chu kX, so hat nh^n X c6n lai 1^:

D.

=

T i so' kho'i Ifftfng cha't Y
•

•

X la:

=

3^.
Aj

=

So hat nhan cua chat Y hinh th^nh la: N„ = N
*

Vdy chon dap an B.

N^A,

^

= -N^.

= 3^.
A,

^

6

G i 4 i c h i l i S t ( J « t h i T S B H , C D M 6 n V$t«

Cdu 11: M o t v a t dao dong dieu hoa c6 chu k i l a T . N e u chon goc thcfi
gian t = 0 l a luc v a t qua v i t r i can bang, t h i t r o n g nufa chu k i dau t i e n ,
van toe cua v a t b k n g k h o n g of t h d i diem:
T
A. t = - .
8

B.t=I.

C.t=

4

T
-.
6

D.

I.
2

Giai
1
Chon goc t h d i gian t = 0 l a luc v a t qua v i t r i can b k n g , t h i — chu
k i nufa v a t se tcri b i e n v a v a n toe b k n g k h o n g . T r o n g nufa chu k i dau
T
tien, v a n toe cua v a t b k n g k h o n g d thdi diem t = —.

4
Vdy chon dap an B.
Cdu 12: M o t l a t h e p mong, m o t dau co d i n h , dau con l a i diJtfc k i c h
t h i c h de dao dong v d i chu I d k h o n g doi va b k n g 0,08 s. A m do l a t h 6 p
phat r a l a :
A. sieu a m .

B. ha am.

C. nhac a m .

D. a m m a t a i ngifdi nghe diftfc.

Giai
T a n so a n . do t h e p p h a t r a : f = - = — ^ = 12,5 < 16Hz.
T 0,08
T a i ngifori k h o n g t h e nghe di/grc a m co t a n so dtfdi 16Hz n e n a m do
la thep p h a t r a l a h a a m .
Vdy chon dap an B.
Cdu 13: P h a t bieu nao sau day l a s a i k h i n 6 i ve do p h o n g x a (hoat d6
phong xa)?
A. Dcfn v i do do phong xa l a beccfren.
B. Do phong xa l a d a i luang

dac trifng eho t i n h p h o n g xa m a n h

hay yeu cua m o t li/cfng chat phong xa.
C. Vdri m o i

iLfotng

chat phong x a xAc d i n h t h i dp p h o n g x a t i le vdri

so nguyen tuf cua liTcfng chat do.
D. E)o phong xa cua m o t lacfng chat ph6ng xa p h y thuoc n h i $ t dp
cua lUtfng chat do.

Giai
Do phong x a cua m o t li/tfng chat phong x a k h o n g p h u thuoc v k o
cdc tac dong b e n ngoai nhtf n h i e t do, ap suat, a n h sang,...
Vdy chon dap an D.
GIJichitiStdlthiTSDH, CDMdn V l L t l f

7


Cdu 14: Doan mach dien xoay chieu khong phan nhanh gom a/^n day
C O do tir cam L , dien t r d thuan R va tu dien c6 dien dung C. K h i dong
dien c6 tan so' goc

chay qua doan mach t h i he so cong suat cua

doan mach nay
A. bang 0.
B. phu thuoc dien trof thuan cua doan mach.
C. b k n g l .
D. phu thuoc tong irb cua doan mach.
GiSi
K h i dong dien c6 tan so goc

chay qua doan mach t h i xay ra

hi^n tiTtfng cong hLfomg. K h i do u, i cung pha va he so cong suat cua
doan mach bang 1.
Vdy chgn dap an C.
Cdu 15: Cho hai dao dong dieu hoa cung phiTcfng, cung tan so, cung
bien do va c6 cac pha ban dau la — va - —. Pha ban dau cua dao dong
3
6
tong hcfp hai dao dong tren bkng:
A. ^ .
12

B. ^ .
6

C.

2

D. ^ .
4

Giai
Pha ban dau cua dao d6ng tong hap hai dao dong tren bang

7t

12

Vdy chgn dap an A.
Cdu 16: Trong mot mach dao dong LC khon^ c6 dien t r d thuan, c6 dao
dpng dien tii ttf do (dao dong rieng). Hieu dien the

CLTC

dai gifla hai

ban tu va ctr&ng do dong dien ciJc dai qua mach Ian Itfcft la Uo va I Q .
T

Tai thdi diem ctfofng do dong dien trong mach c6 gia t r i y

t h i do 16n

hieu dien the giiJa hai ban tu dien la:
A. ^ U o .

C.iu,

B. |Uo.

8

D . ^ U ,

. Gi^i Chi ti^t de thi T S B H , C D IVIon Vat If


Giai

cu

nen W = Wd + Wt O Wt = W - Wa

cu'
2

LI'

LI'
8

2

8

2

Vay chgn dap an D.
Cdu 17: Trong quang pho cua nguyen tuf hidro, neu biet h\idc song dai
nhat cua vach quang pho trong day Laiman la X, i va h\i6c song cua
vach ke vdfi no trong day nay la ^2 thi h\idc song
pho

cua vach quang

trong day Banme l a :

A. (X,-^,)-

c.

B.

+

D.

Giai
Difa vac scf do chuyen mufc nSng
Iifpng, ta c6:
hfi = E i - E o

(1)

hf2 = E2 - Ee

(2)

hf„ = E2 - E l

(3)

E3-

E2-

K

Ei-

TrCf ve vdi ve (2) va (1) ta difpc:

Eo-

hf„ = hf2 - hfi
1-v he ^ he he
V i hf = — nen — =

he

.
=>^„ =

Vay chgn dap an B.
Cdu 18: Phat bieu nao sau day l a s a i k h i noi ve dao dong cua con iSc
dcfn (bo qua liic can cua moi triTdng)?
A. Vdri dao dong nho thi dao dong cua con lac l a dao dong dieu h6a.
B. K h i vat nang d vi tri bien, cd nang cua con lac bang the nang
cua n6.
C. Chuyen dong cua con iSc tCf vi tri bien ve vi tri can bang la
nhanh dan.
D. K h i vat nang di qua vi tri can bang, thi trong

ILTC

tac dung len

no can Vlng vdi life cSng cua day.

Giai Chi l e t d6 thi TS DH, CD Mdn V$t If

9


Khi vat nSng di qua v i t r i can bang, do chuyen dong tren cung tr6n
nen hap life cua P v^ sufc cSng T tao thknh life hirdmg tam. V i v^y,
trong liic t^c dung len no khong can bkng vdri lire cang cua day.
Vay chon dap an D.
Cdu 19: Trong nguyen tuf hidro, Mn kinh Bo la ro = 5,3.10""m. Ban
kinh quy dao dCifng N la:
D. i32,5.10-"m.

C. 21,2.10-"m.

B. 84,8.10-"m.

A. 47,7.10-" m.
Giai

Ban kinh quy dao dCtog N la: r„ = n\ 16ro = 84,8.10-"m.
Vay chon dap an B.
Cdu 20: Mot chat phong xa c6 chu ky ban ra la 3,8 ngay. Sau thcfi gian
11,4 ngay t h i do phong xa (boat do phong xa) cua liTong chat phong xa
con lai bSng bao nhieu phan trSm so v6i do phong xa cua lirang chat
phong xa ban dau?
A. 12,5%.

'
B. 25%.

C. 75%.

D. 87,5%.

V i 11,4 ngay tifdng uTng vdi 3 chu k i T nen dc phong xa con lai la:
H = ^

= ^

= 12,5o/oH,

Vay chon dap an A.

°

Cdu 21: Neu trong mot doan mach dien xoay chieu khong phan nhdnh,
ciicfng do dong dien t r i pha so vdri hieu dien the giUa hai dau doan
mach, t h i doan mach nay gom
A. tu dien va bien t r d .
B. dien tror thuan v^ cupn cam.
C. cuon day thuan cam va tu dien vdi cam k h ^ g nho hcfn dung khang.
D. dien t r d thuan va tu dien.
Neu trong mot doan mach dien xoay chieu khong phan nhanh,
ciTong do ddng dien tre pha so v6i hieu dien the giffa hai dau doan
mach, t h i doan mach nay gom dien t r d thuan va cupn cam.
Vay chon dap an B.
10

GiSi Chi titft

Cdu 22: P h a t bieu nao sau day 1^ s a i k h i n o i ve d n h s^ng dcfn s^c?
A. A n h s4ng dan sic la anh sang khong b i tan sSc k h i d i qua lang k i n h .
B. T r o n g cung m o t m o i trUcrng t r u y e n , v a n t6'c ^ n h sang t i m nho
hcfn v a n t6'c a n h sang do.
C. T r o n g chan k h o n g , c i c d n h sang dcfn s^c khac nhau t r u y e n d i
v d i cung v a n toe.
D . C h i e t suat cua m o t m o i tri/ofng t r o n g suot doi v6i d n h sdng d6
\&ti hon chiet suat cua m o i t n i d n g do doi vdri a n h sang t i m .
Doi v d i a n h sdng dcfn s^c, chiet suat cua m o t m o i triTdng t r o n g su6't
doi vdri a n h sang tSng len tiif do den t i m . V i vay chiet suat cua d n h sdng
do p h a i nho hcfn chiet suat cua m o i triicfng do d6'i v d i a n h sang t i m .
Vay chgn dap an

B.

Cdu 23: Cho doan mach g6m dien trdr t h u a n R n o i t i e p vdri t u d i e n c6
dien dung C. K h i dong dien xoay chieu c6 t a n so goc (o chay qua t h i
tong trdr cua doan mach l a :
A. ^ R ' + ( c o C ) ' .

B.

|R' +

B. ^ R ' - ( a ) C ) ' .

D. . R' -

1

coC

Giai
T o n g trcf cua doan mach l a : Z = ^R^ +
Vdy chgn dap an

=

R^ +

J

B.

Cdu 24: Theo t h u y e t Ufgrng tuf a n h sang t h i nSng liTOng cua
A. m g t p h o t o n t i le t h u a n v d i biTdc song a n h sang tiTcfng l i n g v d i
photon do.
B. m o t p h o t o n b^ng nSng liigfng n g h i cua m o t electron

(electron).

C. m o t p h o t o n p h u thuoc vao khoang cdch tCr p h o t o n do t d i nguon
p h d t ra no.
D n S c p h o t o n t r o n g chum sang dcfn s^c b^ng nhau.
Theo t h u y e t liTcfng tijf a n h sdng t h i nSng 'k/dng cua cac

ph6t6n

t r o n g chum sdng dcfn s^c b k n g nhau.
Vdy chgn dap an D.
GAi C h i t\6x ai thi

O H , C O M6n V $ t If

11


Cdu 25: H i e u d i e n t h e giffa ano't v a cato't cua m o t o'ng Rornghen l a
U = 2 5 k V . Coi v a n toe ban d a u cua chum electron (electron) p h a t r a ttf
catot b k n g k h o n g . B i e t h a n g so P l a n g h = 6,625.10~^^J.s, d i e n t i c h
nguyen to b a n g 1,6.10'^^C. T a n so Idm n h a t cua t i a Rornghen do ong
nay c6 the p h a t r a l a :
D. 60,380.10'^Hz.

C. 6,038.10^^Hz.

B. 60,380.10^^Hz.

A. 6,038.10^^Hz.

'

Giai
PT J

Ta c6: e U = h f = > f = — = 6,038.10"HZ.
h

Vdy chgn dap an A.
Cdu 26: M o t k h u n g d a y d a n h i n h chOr n h a t c6 100 vong, d i e n t i c h moi
vong 600 cm^, quay deu quanh true doi xufng cua k h u n g vdfi v a n toe goe
120 vong/phut t r o n g m o t tCf truf&ng deu eo cam ufng ttf bang 0,2T. True
quay vuong goe v d i cac dUorng cam ufng tCr. Chon goe thori gian la lue
vectof phap t u y e n cua m a t phSng k h u n g d a y ngUtfc hifdfng vdfi vector cam
ufng tii. B i e u thufc suat d i e n dong cam ufng t r o n g k h u n g l a :
A . e = 4,8;isin 4 0 ; r t - - ( V ) .
2
C. e =

4871

B. e = 487tsin(4;rt + n ) ( V ) .

sin 4 0 7 r t - - ( V ) .
2

D. e = 4,87isin(407rt + 7 i ) ( V ) .

Giai
Theo de b a i , t a c6:
+ D i e n t i c h v o n g day: S = 600cm^ = 6.10"^m^
+ So v o n g day: N = 100 vong.
•f C a m ufng tCr: B = 0,2T.
+ Tdc do quay cua roto: 120 vong/phut = 2 vong/giay.
Vao thcfi d i e m t = 0, goe giffa phap t u y e n cua k h u n g day va cam
ufng tff l a a = 71.
Ta C O tCf t h o n g qua m o t vong day l a :
cp, = BScos(a + 7r) = BScos(27rnt + 7r).

V i k h u n g day c6 N vong giong nhau n e n t r o n g k h u n g c6 suat dien
dong: e = -

^ = NBS27msin(27mt + 7 r ) .

N

T h a y so t a c6: e = 4,87isin(407rt + 7 i ) ( V ) .
Vdy chgn dap an D.
12

Giai Chi ti^t dS thi T S DH, C D M6n V^t If


Cdu 27: T r o n g t h i n g h i e m giao thoa a n h sang vdfi khe l a n g (Y-ang),
khoang each giffa h a i khe la 2 m m , khoang each tii m a t p h a n g chufa
hai khe den m a n quan sat la 1,2 m. Chieu sang h a i khe b a n g a n h sang
hon hop gom h a i a n h sang dcfn sSc c6 badfc song 500 n m va 660 n m
thi thu

diXac

h§ v a n giao tlioa t r e n m a n . B i e t v a n sang ehinh

giUa

(trung tarn) ufng vdri h a i bufe xa t r e n t r u n g nhau. K h o a n g each tii v a n
chinh giffa den v a n gan n h a t cung mau v d i v a n c h i n h giufa l a :
A. 9,9 m m .

B. 19,8 m m .

C. 29,7 m m .

D. 4,9 m m .

Giai
T a c6:

vdri X. = 500nm t h i i , =

a

= 0,3mm.

vdri ^2 = 660nm t h i
k i i i = k2i2

= - ^ = 0,396mm.
a
k| _ i , _ 396 _ 33

k^ " i j ~ 300 " 2 5 '

Gia t r i nguyen nho n h a t la k i = 33, k2 = 25.
Khoang each tii v a n ehinh giffa den v a n gan n h a t cung m a u vdfi
v a n ehinh giffa l a : 33.0,3 = 9,9mm.
Vay chqn dap an A.
Cdu 28: M o t con lac 16 xo gom 16 xo eo do eijfng 20 N / m va v i e n b i c6
kho'i lirgrng 0,2 k g dao dong dieu hoa. T a i t h 6 i d i e m t , v a n to'c va gia

toe eua v i e n b i I a n li/gtt l a 20 cm/s va 23 m/s^. B i e n do dao dong cua
vien b i l a :
A. 4 em.

B. 16 em.

C. 10 V3 em.

D. 4 V 3

cm.

Giai
A p dung d i n h l u a t bao toan nang iLfgrng, t a c6:

i k A ^ = - k x ^ + - m v ^ =-!-kx^ +-\w'
2
2
2
2
2co'
Thay cAc gia t r i co -

=> A = .

V

CO

— = 10(rad/s), v = 20(cm/s), a = -co^x vao bieu

m

thdre t r e n t a di/dc A = 4cm.
Vdy chgn dap an A.
Cdu 29: M o t con l^c 16 xo treo thSng dufng. K i c h t h i e h cho con l^c dao
dong dieu hoa theo phiTOng t h a n g dufng. Chu k i va b i e n do dao dong
GiaiChitiStdSthiTSDH, CDM6nVatli

•

•

13


cua con He I a n

lifOt

l a 0,4

s

va 8 cm. Chon

true

x'x t h i n g

duTng

chieu

ducrng hifdng xuong, goc t o a do t a i v i t r i can b k n g , goc t h 6 i gian t = 0
k h i v a t qua v i t r i c a n b ^ n g theo c h i e u difdng. L a y g i a to'c r o i tii do
g = 10 m/s^ v ^ 7t^ = 10. Thori gian ng^n n h a t ke tijf k h i t = 0 den k h i
ILTC

d a n h o i cua 16 xo c6 do I6n ale t i e u l a :
A. ^ s .
30

B.

±s.

15

C.

Is.

D.

10

-s.
30

GiSi

Chu k i : T =

2%

CO = STC.

CO

K h i treo v a t vko, 16 xo g i a n m o t doan:
A / = — g = = 0,04m < A = 0,08m.
k
CO
LLTC

d^n h o i cua 16 xo c6 do l
K h i do F = 0.
K h i t = 0 t h i X = 0 n e n A sin (p = 0. K e t htfp v d i v > 0 t a c6 phuomg
t r i n h dao dong:
X = 8 sin 57rt => 8 sin 57rt = - 4

sin 57rt

=-0,5

57rt = - — + 2 k 7 t

6
Vdy chon dap an A.
Cdu 30: D a t vao h a i dau doan mach dien R L C k h o n g p h a n n h a n h m o t

hieu d i e n t h e u = 220V2cos c o t - i ^ ( V ) t h i C L r 6 n g do d6ng dien qua
2
doan mach c6 bieu thiJc l a i = l-Jlcos

4

( A ) . Cong sua't tieu t h u

cua doan mach n a y l a :
A. 2 2 0 V 2 W .

B. 440W.

C. 4 4 0 V 2 W .

D . 220 W .

G\k\

Dp lech pha giuTa ciTdng do dong di^rt v a h i e u dien t h e :

V2
^2

n
^ 4
14

Giai chi.tigf 66 thi TS DH, CD M6n VHt II

Cong suat t i e u t i i u cua doan mach:

P = U I cos

Vdy chgn dap an A.
C&u 31: Doan m a c h d i e n xoay chieu g o m b i e n t r d R, cuon day t h u a n
cam CO do tU c a m L v a t u dien c6 dien dung C mSc n o i t i e p . B i e t hi$u
dien the h i e u d u n g giffa h a i dau doan mach l a U , c a m k h a n g Z L , dung
k h a n g Zc (vdfi Zc

Z L ) v a t a n so dong dien t r o n g m a c h k h o n g d d i .

Thay d d i R den gia t r i Ro t h i cong suat t i e u t h u cua doan m a c h d a t gik
t r i ctfc d a i Pm, k h i d6:
A. R o = | ^ .

B. R„=|Z,-Zc|.

C. P . = ^ .

D. R o = Z , 4 - Z e .
Giai

T r o n g doan m a c h c h i c6 R t i e u t h u d i e n nSng diTdi d a n g nhi§t
nang, cong suat t i e u t h u cua doan mach l a :
P = R I ' =R^+(ZL-ZC)

R,(ZL-Zcr

M

R

Ta c6: P =

k h i M = M^^^.

Theo b a t d a n g thufc Cosi t h i : RQ = |ZL -

.

Vdy chon dap an B.
Cdu 32: P h a t bieu nao sau day l a d u n g k h i n o i ve quang phd?
A. Quang p h d h a p t h u 1^ quang p h d cua a n h sang do m o t v a t r ^ n
phat r a k h i v a t do difdc n u n g nong.
B. De t h u difcfc quang p h d h a p t h u t h i n h i e t do cua d a m k h i h a y
hcfi hap t h u p h a i cao hcfn nhiSt do cua nguon sang p h a t r a quang p h d
l i e n tuc.
C. Quang p h d l i e n tuc cua ngudn sang nao t h i p h u thuoc t h a n h
phan cau tao cua nguon sang S'y.
D. M o i ngxiyen to hoa hoc of t r a n g thAi k h i hay hcd nong sang dvCdi ap
suat t h a p cho m o t quang p h d vach r i e n g , dSc trUiig cho nguyen t o do.
Giaichiti§td4thiTSDH. CDM6nV4tl(

'

15


Giai

Moi nguyen to hoa hoc d trang thai k h i hay hofi nong sang diTdfi ap
suat thap cho mot quang pho vach rieng, dac trttog cho nguyen to do.
Vdy chgn dap an D.
Cdu 33: Cho doan mach dien xoay chieu gom cuon day mSc no'i tiep
v(Ji tu dien. Do lech pha cua hieu dien the giCfa hai dau cuon day so v6i
CLfdng

do dong dien trong mach la y . Hieu dien the hieu dung giffa

hai dau tu dien bang Vs

Ian hieu dien the hieu dung giffa hai dau

cuon day. Do lech pha cua hieu dien the giiJa hai dau cuon day so \6i
hieu dien the giffa hai dau doan mach tren la
A. ^ .
3

B.O.

C. ^ .

D.
3

2

Gi^i
Do lech pha cua hieu dien the giUa hai dau cuon day so v6i ci/crng
do dong dien trong mach la ^ nen suy ra:

tan (p, = ^
'

R

tan - = 7 3 => Z, = R V3.
3

Ta c6: Uc = V3 Ud ^ IZ^ = yfz.l^R'+3Zl

= V 3 . W R ' + 3 R ' = 1.2^.R

=^ Zc = 2 V 3 . R

Do lech pha giffa u va i :
tan(p=

Z L - Z C

=

N/3R-2V'3R

r-

71

= _V3=>cp = - y .

Do lech pha cua hieu dien the giCifa hai dau cuon day so vdfi hieu

dien the giCifa hai dau doan mach tren la:
71

271

3
Vdy chon dap an A.
Cdu 34: Trong mach dao dong LC c6 dao dong dien tCr tiT do (dao dong
rieng) vdfi tan so goc lO"* rad/s. Dien tich

dai tren tu dien la lO'^C.

CLTC

Khi cUofng do dong dien trong mach bkng 6.10"^ A t h i dien tich tren tu
dien la:
A. 4.10"^°C.

B. 6.10"^°C.

C. 2.10-^°C.

16

.

-

D. 8.10-^°C.

QiaichitigtdethiTSDH, CBMSn vatli


Theo d i n h luat bao toan nSng lucfng, t a c6:
2

- +2C— cu^
=-

q = ^ C ' U ^ - C L i ' =8.10-'° (C).

Vdy chgn dap an D.
Cdu 35: H a t n h a n ^^gRa bien ddi t h a n h h a t n h a n ^g^R" do p h o n g xa:
A. p \. a va p " .

C. a .

Ta CO phircJng t r i n h : -fgRa ^

.

D. p".

'g^Rn+ 2He. ^He la h a t a.

Vdy chgn dap an C.
Cdu

36:

M o t c h a t d i e m dao d o n g d i e u h o a

X = 3sin

6J

theo phiTcfng t r i n h

(x t i n h bang cm va t t i n h b ^ n g giay). T r o n g m o t giay

dau t i e n tCr thcfi d i e m t = 0, chat d i e m d i qua v i t r i c6 l i do x = + 1cm:
A. 4 I a n .

B. 7 I a n .

C. 5 I a n .

D. 6 Ian. ,

GiSi
Chu k i dao dong: T = — = 0,4s.

N g h i a la t r o n g I s , v a t thufc h i e n

CO

2,5 chu I d . T r o n g m o t chu k i , v a t d i qua v i t r i c6 U do x = + l c m h a i l l n
nen t r o n g I s , v a t d i qua v i t r i c6 l i do x = + l c m n a m I a n .
Vdy chgn dap an C.
Cdu 37: M o t song ccf Ian t r u y e n t r e n m o t dLforng t h i n g tii d i e m O den

diem M each O m o t doan d. B i e t t a n so f, budc song X vk b i e n do a
cua song k h o n g doi t r o n g qua t r i n h song t r u y e n . N e u phi/cfng t r i n h dao
dong cua p h i n tuT v a t chat t a i d i e m M c6 dang UM(t) = asin27tft t h i
phifofng t r i n h dao dpng cua p h a n tuT v a t chat t a i O 1^:
A. U o ( t ) = asinjr

B. U o ( t ) = asin;c

C. U o ( t ) = asin23i

D. U o ( t ) = asin2;i

ft4

Neu phifcfng t r i n h dao dong cua p h a n tijf v a t chat t a i d i e m M c6
#
d
dang U M ( t ) = a sin Inft t h i t h d i gian t r u y e n song tCf O d§n M 1^ At = —.
Gt&i Chi ti^t d l thi TS OH. CD M6n V$t II

I

17


Khi do dao dong tai O sdm pha hon tai M la

27rd

.

Phi/ong t r i n h dao dong cua phan tuf vat chat tai O la:
Uo(t) = asin27r

Vdy chqn dap an C.
Cdu 38: K h i c6 hien ttfcrng quang dien xay ra trong te bao quang dien,
ph6t bieu nao sau day la sai?
A. Giuf nguyen

cLforng

do chum sAng kich thich va k i m loai dung

lam catot, giam tan so cua anh sang kich thich t h i dong nang ban dau
C L T C

dai cua electron (electron) quang dien giam.
B. GiCr nguyen chum sang kich thich, thay ddi k i m loai lam catot

thi dong nang ban dau ciTc dai cua electron (electron) quang dien thay doi.
C. Giuf nguyen t a n so cua anh sang kich thich va kim loai lam
catot, tang cifcfng do chum sang kich thich t h i dong nang ban dau c\Xc
dai cua electron (electron) quang dien tSng.
D. Giu" nguyen ctfcfng do chum sang kich thich va kim loai dung
lam catot, giam hiidc song cua anh sang kich thich t h i dong nSng ban
dau

CLfc

dai cua electron (electron) quang dien tang.

Giai
Theo dinh luat quang dien, khi c6 hien tiigrng quang dien xay ra
trong te bao quang di?n, dong nang ban dau ciic dai ciia cac quang
electron hoan toan khong phu thuoc vao ciicfng do anh sang kich thich
ma chi phu thuoc vao budfc song va tan so.
Vdy chgn dap an C.
Cdu 39: Tai hai diem A va B trong mot moi tri/dng truyen song c6 hai
nguon song ket hop, dao dong cung phtrong vdri phifcfng trinh Ian lifc*t
la UA = asincot va U B = asin((ot + 7 i ) . Biet van toe va bien do song do
m5i nguon tao ra khong doi trong qua trinh song truyen. Trong khoang
giuTa A va B CO giao thoa song do hai nguon tren gay ra. Phan tuf vat
chat tai trung diem cua doan AB dao dong vdfi bien do bkng:
A. - .

B. 2a.

C. 0.

D. a.

2
18

GiSi Chi tigt

thi TS D H , C D M6n VSt!(


Giai

T a i t r u n g diem dao dong eua h a i nguon t r u y e n tori ngi/gfc pha nhau
nen bietn dp dao dong t o n g hgfp bSng 0.
Vay chgn dap an C.

.

Cdu 40: P h a t bieu nao sau day la dung k h i n o i ve dong d i e n xoay
chieu ba pha?
A. K h i cifcfng do dong dien t r o n g m o t pha b k n g k h o n g t h i cifdng do
dong dien t r o n g h a i pha con l a i khac k h o n g .
B. Chi C O dong dien xoay chieu ba pha m d i tao diic/c tCf tn/c/ng quay.
C. K h i ciidng do dong dien t r o n g m o t pha ctfc d a i t h i ci/cfng do
dong dien t r o n g h a i pha con l a i ctfc tieu.
D. Dong dien xoay chieu ba pha la h e t h o n g gom ba dong dien
xoay chieu m o t pha, lech pha nhau goc

2n
—.

Giai

Dong dien xoay chieu ba pha la he t h o n g gom ba dong d i e n xoay
chieu m o t pha, lech pha nhau goc
Vay chon dap an D.
Cdu 41: Neu chum t i a sang 16 k h o i t h a u k i n h p h a n k y ma h o i t u t a i
mot diem t h i chum t i a td\u k i n h do c6 diTdng keo d a i :
A. giao nhau cf sau t h a u k i n h va giao d i e m each t h a u k i n h m o t
khoang nho h o n do Idrn t i e u C L T cua t h a u k i n h .
B. giao nhau d sau t h a u k i n h va giao d i e m each t h a u k i n h m o t

khoang Idfn hon do Idfn t i e u C L T cua t h a u k i n h .
C. giao nhau of sau t h a u k i n h va giao diem t r u n g vdri t i e u d i e m v a t
cua t h a u k i n h .
D. song song vdfi true c h i n h cua t h a u k i n h .
Giai

Neu chum t i a sang 16 k h o i t h a u k i n h p h a n k y m a h o i t u t a i m o t
diem t h i chum t i a t 6 i t h a u k i n h d6 c6 dudng keo d a i giao nhau 6 sau
thau k i n h va giao diem each t h a u k i n h m o t khoang nho hOn do 16n
tieu C U T ciia t h a u k i n h .
Vay chgn dap an A.
Giai Chi tietde thi TS D H , C D Mon Vat li

,
'

19


Cdu 42: Mach dao dong cua mAy thu song v6 tuyen c6 tu dien vdung C va cuon cam \6i do tU cam L , thu dUtfc song dien tii c6 biTdfc
song 20 m. De thu difac song dien t\i c6 hxicSc song 40 m, ngifcfi ta phai
m^c song song vdri tu dien cua mach dao dong tren mot tu dien c6 dien
dung C b^ng:
A. 4C.

B. 3C.

C. C.

D. 2C.

G\h\
Bi/dfc s6ng mdy thu k h i chi c6 tu C :
Bircfc song may thu k h i tu C // C:

C.27IVLC = 20.
= c.ln^LCl

= 40.

Taco: — = - = ^ C j =4C.
C,
4
'
K h i m^c song song t h i ta c6: C = C2 - C = 4C - C = 3C.
Vay chgn dap an B.
Cdu 43: K h i tinh tien cham mot vat AB c6 dang doan t h i n g nho doc
theo va luon vuong goc vdri true chinh (A n^m tren true chinh) cua mot
mit khong c6 tat tii xa den diem cufc can cua no, t h i c6 anh luon hien
ro tren vong mac. Trong k h i vslt dich chuyen, tieu CLT cua thuy tinh the
v^ goc trong vat cua m^t nay thay doi nhii the nao?
A. Tieu ciT tkng, goc trong vat tSng.
B. Tieu cU giam, goc trong vat tSng.
C. Tieu cU giam, goc trong vat giam.
D. Tieu cU tang, goc trong vat giam.
GiSi

Chon dap an B.
Cdu 44: Cho mot he hai thau kinh mong L i va L2 dong true chinh. L i

la thau kinh hoi tu c6 tieu ciT 12cm. Tren true chinh, tri/dc L i dSt mpt
diem sdng S cAch L i Ik 8cm. Thau kinh L2 dSt tai tieu dien anh cua
L i . De chum sang phat ra tCf S, sau k h i qua he la chum song song vdri
true chinh t h i do tu cua thau kinh L2 phai c6 gia t r i :
A. — diop.

3

B. - diop.

C. — diop.

2

9

D. - diop.

3

Giai

Diem sAng S qua L i cho anh S' each L i mot doan:
d ; = ^ = -24(cm).
20

G i i i Chi tifi'l

thi TS DH. CD Mfln V a t If

D i l m sang S qua L 2 cho a n h S ' each L 2 m o t doan:
da = 12 + 2 4 = 3 6 (cm).
Muon chum t i a qua L 2 la chum song song t h i S ' p h a i n a m of t i e u
diem F2 cua L 2 , hay t i e u cU cua L 2 la £2 = da = 3 6 c m = 0,36m.
1
1
25
.
Dp t u cua t h a u k i n h L 2 p h a i c6 gia t r i : D = j = Q~3^ ~ "9" (^^°P^Viay chon dap an C.
Cdu 45: M o t k i n h h i e n v i quang hoc gom v a t k i n h v a t h i k i n h c6 t i e u
cLf

I a n luot l a 0,5 c m v a 4 cm. K h o a n g each giufa v a t k i n h v a t h i k i n h

b^ng 2 0 c m . M o t ngUdi m ^ t k h o n g c6 t a t , c6 d i e m ctfc « a n each m ^ t
25 cm, suT dung k i n h h i e n v i nay de quan sat m o t v a t nho. D o b p i giac
cua k i n h k h i n g ^ m ehCrng d v6 cufc la:
A. 19,75.

B . 25,25.

C . 193,75.

D . 250,25.

Giai
T a c6: D = 2 5 c m , fi = 0,5cm, {2 = 4 c m , 5 = 2 0 - 4,5 = 15,5cm.
Do boi giac cua k i n h k h i ngim

ehCfng d v6 cifc 1^: G =

= 193,75.
hh

Vay chgn dap an C.
Cdu 46: M o t t i a sang dcn sic t r u y e n tii m o i trir&ng (1) c6 ehiet suat
tuyet doi n i sang m o i trifcfng (2) c6 ehiet suat tuyet doi na t h i t i a k h i i c
xa lech x a phap t u y e n hcfn t i a tdi. H i e n ti/png p h a n x a t o a n p h a n c6
the xay r a k h o n g neu chieu t i a sang theo chieu tii m o i trifcfng (2) sang
moi trifcfng (1)?
A. K h o n g t h e , v i m o i triX&ng (2) ehiet quang hcfn naoi triTcfng (1).
B. C o t h e , v i m o i trifofng (2) chie't quang k e m m o i triTcfng (1).
C . C o t h e , v i m o i trifcfng (2) ehiet quang hcfn m o i triTcfng (1).
D. K h o n g t h e , v i m o i trUcfng (2) ehiet quang k e m m o i tri/orng (1).

Giai
K h o n g t h e , v i m o i tnicfng (2) ehiet quang k e m m o i trifcfng (1).
Vay chgn dap an D.
Cdu 47: M o t ngifcfi can t h i c6 d i e m cife can c ^ c h m ^ t 10 c m , d i e m ciic
vi§n each m ^ t 50 cm, deo k i n h eo do t u - 2 diop s 6 t mAt t h i n h i n ro vat:
A. each mkt 5 0 c m m ^ mSt k h o n g can dieu t i e t .
B. d gan n h a t cdch m ^ t m o t doan 10 cm.
C . or x a v6 ciTe nhiftig m ^ t v a n can dieu t i e t .
D. d x a v6 cue m a m ^ t k h o n g can dieu t i e t .
GtSi Chi t i ^ t

t h i T S D H , C D M6n V $ t If

21

mat

NgLfcfi c a n t h i
50

xa v6

c m , deo

C L T C

m a

c6

diem

k i n h c6

mat

do

C L T C

Giai
can each mat

tu -2

10

diop sat m a t

c m , d i e m ciic v i l n
t h i c6

the

n h i n ro

each
vat

of

k h o n g c a n dieu tiet.

Vay chqn dap an D.
Cdu 48: V a t sang A B c6 dang m o t doan thSng, dat vuong goc vdi true
chinh ( A n k m t r e n true chinh) cua mot thau k i n h , tao ra anh A i B i = 4em
ro n e t t r e n m a n . Giuf v a t va m a n co d i n h , d i chuyen t h a u k i n h doc
theo true c h i n h den m o t v i t r i khac t h i l a i t h u diTOc a n h A2B2 = 6,25cm
r5 net t r e n m a n . Do cao v a t A B bkng:
A . 1,56 cm.

B . 25 cm.

C. 5 cm.

D. 5,12 cm.

Giai
T a c6 2 v i t r i :
... _ . d
A'B'
4
- V i t n l : — =
=
d'
AB
AB

_ V i t r i 2 : l =^
d'

A"B"

(1)

=^

(2)

6,25

G i a i he gom h a i phifcfng t r i n h (1) va (2) ta duac A B = 5.
Vay chon dap an C.

Cdu 49: M o t tha'u k i n h m o n g bang t h u y t i n h h a i m a t cau l o i , c6 chiet
suat tuyet doi n . T h a u k i n h nay c6 do t u :
A . a m k h i d a t t r o n g m o i triicfng c6 chiet suat tuyet doi n ' < n .
B . difcfng k h i dat t r o n g m o i trUofng c6 chiet suat tuyet doi n ' = n .
C. a m k h i dat t r o n g m o i tri/ofng c6 chiet sua't tuyet doi n ' > n .
D. luon dUtfng, k h o n g phu thuoc vao m o i tn/cfng chufa thau k i n h .

G\k\
M o t t h a u k i n h m o n g hkng

t h u y t i n h h a i m a t cau l o i , c6 chiet suat

tuyet doi n . T h a u k i n h nay c6 do t u a m k l i i dat t r o n g m o i trUcfng c6
chiet suat tuyet doi n ' > n .
Vay chgn dap an C.
Cdu 50: M o t k i n h t h i e n v a n quang hoc gom v a t k i n h va t h i k i n h l a cAc
t h a u k i n h h p i t u c6 t i e u CLT I a n li/crt la 1,2 m va 6 cm. M o t ngifcfi m^t
k h o n g C O t a t , quan sdt m p t t h i e n the of r a t xa hkng k i n h t h i e n v a n nay
t r o n g t r a n g t h a i m ^ t k h o n g dieu t i e t c6 goc t r o n g a n h la 5'. Goc

trong

t h i e n the k h i k h o n g d u n g k i n h l a :
A . 0,5".

B . 0,25'.

C. 0,35',

22

•

D. 0,2'.
GAi Chi ti^t 66 thi TS BH,

C B

M6n V^t l(


Giai
Do boi giac cua k i n h l a : G = ^ = —
= 20.
•
0,06
Goc t r o n g t h i e n t h e k h i k h o n g dung k i n h l a :

=^

=

= 0,25'.

Vay chon dap an B.
Cdu 51: M o t v a t r a n quay quanh m o t true co d i n h d i qua v a t co phtfcfng
t n n h chuyen dong (p = 10 + t^ (cp t i n h bang r a d , t t i n h b k n g giay). Toe
do goc va goc m a v a t quay ducfc sau t h d i gian 5 s ke tCf thcfi d i e m t = 0
Ian lirgft l a :
A. 5 rad/s va 25 r a d .

B. 5 rad/s va 35 r a d .

C. 10 rad/s va 35 r a d .

D. 10 rad/s va 25 r a d .

Giai
So sanh phifdng t r i n h cp = 10 + ( c p

t i n h b k n g rad, t t i n h b k n g giay)

vdi phifotng t r i n h t o n g quat (p = % + (ii^t + ^yt^ t a difdc cac gia t r i :
(Po = lOrad/s; © 0 = 0;

= 1 =:> Y = 2rad/s\

M a t khac, phicfng t r i n h toe do goc: co = yt •
Sau 5s, t a co:

co = 2.5 = 10 rad/s.
A(p = (p-cpo = 1 0 + t ^ - 1 0 = 25rad.

Vdy chgn dap an D.
Cdu 52: M o t dia p h i n g dang quay quanh true eo d i n h d i qua t a m v ^
vuong goc vcri m a t p h a n g dia v d i toe do goe k h o n g d d i . M o t d i e m b a t
ky n k m d mep dia
A. k h o n g CO ca gia toe hudtng t a m va gia toe t i e p t u y e n .
B. CO ca gia tdc hudfng t a m va gia toe t i e p t u y e n .
C. c h i c6 gia toe hifdng t a m m a k h o n g co gia tdc t i e p t u y e n .

D. c h i CO gia tdc t i e p tuyen m a k h o n g co gia tdc hirdrng t a m .

GiSi
M o t diem ba't k y n k m d m6p dia chi co gia tdc hifdng t a m m ^
k h o n g CO gia tdc t i e p tuyen.
Vdy chon dap an C.
Giii chi tiSt di thi TS DH, CD M6n V$t l(

23


I

C&u 53: Mot r6ng roc c6 true quay nkm ngang co dinh, ban kinh R,
khoi lUcfng m. Mot soi day khong dan c6 khoi lUcfng khong dang ke,
mot d^u quan quanh rong roc, dau con lai treo mot vat khoi liTOng
cung bkng m. Biet day khong trifot tren rong roc. Bo qua ma sat cua
rong roc vdi true quay va sure can cua moi trirfirng. Cho momen quan
tinh cua r6ng roe doi vdi true quay la

va gia toe rdi t i i do g. Gia

t6c cua vat k h i dugc tha rcfi la:
A. ^ .
3

B. ^ .
3

C. g.

^

D. ^ .
2

Gi^i

(3)

Ta lai c6: a = yR

(2)

Ap dung dinh luat eo ban cua d6ng liTe cho vfit: mg - T = ma

(1)

Ap dung dinh luat co ban eua dong life cho rong roe: TR = ly

Giai h$ gom 3 phiTOng trinh (1), (2) va (3) ta

dLTgrc: ^

=

Vdy chgn dap an A.
Cdu 54: Mot thanh manh AB dong chat tiet dien deu, chieu dai A,
khoi lifcfng m. Tai dau B cua thanh ngifdi ta g^n mot chat diem co khoi
luong

Kh6'i tam cua h$ (thanh v^ chS't diem) each dau A mot doan:

£
A. - .
2

£
B. - .
6

U
C. — .
3

i
D. - .
3

Kh^i tam cua h$ (thanh va cha't diem) cdch dau A mpt doan — .
V&y chgn dap an C.
Cdu 55: Mpt thanh manh dong chS't tiet dien deu, khoi liTOng m, chieu
d^i A, e6 the quay xung quanh true nam ngang di qua mot dau thanh
v^ vuong goc vdi thanh. Bo qua ma sat d true quay va sure can eua moi
tn/dng. Momen qudn tiniA eua thanh doi vdi true quay la I = -jmf^ va
gia toe rdi tiT do la g. Neu thanh dUde tha khong van toe dau tCf v i t r i
nkm ngang t h i k h i tdi v i t r i thang ddng thanh co toe do g6c co b^ng:

A.S.
\2e

C.M.

B.,S

\i

V3£

24

oai

D.K
hi

Chi ti«t (3« thi T S D H , C D M6n V$t If