Cdu 31: U m a t chat l o n g c6 h a i nguon song A, B each 18 cm, dao dong theo phtrcfng t h S n g duTng v d i phMng t r i n h la U A = U B = acosSOTit iv6i t t i n h b a n g s) Toe t r u y e n song cf m a t chat l o n g la 50 cm/s Goi O l a t r u n g d i e m eua A B , d i e m M or m a t chat l o n g n k m t r e n diTdng t r u n g trtfc cua A B va gan O n h a t cho p h a n tuf chat long t a i M dao dong cung pha v d i p h a n tuf chat l o n g t a i O K h o a n g each M O l a : A 10 cm B cm C V c m , D V i c m Giai d ^ ^ A B Phi/cfng t r i n h song t a i m o t diem M t r e n di/ofng t r u n g triTc va diem Ola: u^^ = 2a Acp^ =9TI Ta c6: COS 507rt 27id X dmin k h i va 27ld^ ; U Q = 2acos(507rt-97c) = 2k7i=>d = - k > A O = ^ k < chi k^ax = suy -1, dmin = 11 K h i M O = d L - A ^ = 2710 Vdy chgn dap an D 32: M o t s^i day d a n h o i cang ngang, dang c6 song diTng on d i n h Cdu T r e n day, A l a m o t d i e m n u t , B la m o t diem bung g a n A n h a t , C la t r u n g d i e m cua A B , v6i A B = 10 cm B i e t khoang t h d i gian ng^n n h a t giufa h a i I a n m a l i dao dong cua p h a n tuf t a i B bang b i e n dao dong cua p h a n tuf t a i C l a 0,2 s Toe t r u y e n song t r e n day l a : A , m / s B , m / s C m/s D m/s Giai T a c6 b i e n song dCfng t a i m o t d i e m M t r e n day, each dau co d i n h A doan d l a : A M 27id = a smvdi a l a b i e n nguon song X 120 G i l i i chr t i g t d S thi TS B H , C D M o n V$t II - B i e i i song t a i d i e m B AB = d„ = - = 10=>> = c m " 2a - Bien s6ng t a i diem C 271- Ac = 2a sm dc = AB I ~8 V2 = 2a.—= A, 37t 71 V i GO the coi d i e m B nhtf m o t chat diem dao dong dieu hoa vori bien A B , t h i t h d i gian ngan n h a t giOfa h a i Ian diem B c6 l i ; ABI 331 T At = - = , ^ T Vgy chon dap an X = 0,8s=>v = - - , m / s T / \ — \ _ Jt r n B Cdu 33: M o t nguon diem O p h a t song a m c6 cong suat k h o n g doi t r o n g mot m o i t r i f d n g t r u y e n a m dang hifdng va k h o n g hap t h u a m H a i diem A, B each nguon a m I a n Ixiat la ri va r2 B i e t ci^cfng a m t a i A gap Ian ci/&ng a m t a i B T i so — bang: A B C - D - Giai A p dung cong thufc I I Vdy chon dap an A Cdu 34: D a t dien ap u = UV2 coscot vao h a i dau m o t t u d i e n t h i cUcfng dong dien qua no c6 gia t r i hieu dung la I T a i t h o i d i e m t , d i e n ap Giai Chi jigt de ti-.i TS DH, CD M6n Vat li 121 d h a i d a u t u d i e n 1^ u va cifcfng dong d i e n qua no l a i He thijfc lien h e giiJa c a c d a i lu"crng l a : -2 "> -2 Giai V i uc v u o n g p h a i n e n giOfa c h u n g luon c6 m o t cong thuTc doc l a p -2 thcfi g i a n : — — + — = Suy r a : ^ ^ — y + — — ^ = => — + - = Vdy chgn dap an D Cdu 35: M o t hoc s i n h quan m o t m a y b i e n ap v6i d i i d i n h so vong day cua cuon sor cap gap h a i I a n so vong day ciia cuon thuf cap Do scf suat n e n cuon thuT cap b i t h i e u m o t so voiig day M u o n xac d i n h so vong j day t h i e u de quan t i e p t h e m vao cuon thuf cap cho du, hoc sinh dat \ vao h a i dau cuon scf cap m o t d i e n ap xoay chieu c6 gia t r i hieu dung k h o n g doi, r o i d u n g v o n ke xac d i n h t i so dien ap d cuon thtr cap de hof va cuon scf cap Liic dau t i so dien ap b k n g 0,43 Sau k h i quan them vao cuon thuf cap 24 v o n g day t h i t i so dien ap bang 0,45 Bo qua moi hao p h i t r o n g m a y b i e n ap De disac m a y b i e n ap diing nhif diT d i n h , hoc s i n h n a y p h a i t i e p tuc quan t h e m vao cuon thuf cap: D 40 vong day C 60 v o n g day B 84 vong day A 100 v o n g day Gihi Goi N i , N2 l a so v o n g day ban dau cua m i cuon; n l a so vong day p h a i cuon t h e m T a c6: N N +24 ^ = 0,43; = 0,45 N, N, ^' N2+24 +n N , = 1200; N , = 516 ' ' = ^ n = 60 Vdy chgn dap an C 122 GiaJchitietdethiTSDH,CDM6nVatli ...- B i e i i song t a i d i e m B AB = d„ = - = 10=>> = c m " 2a - Bien s6ng t a i diem C 27 1- Ac = 2a sm dc = AB I ~8 V2 = 2a.—= A, 37t 71 V i GO the coi d i e m B nhtf m o t chat diem dao... n ap Giai Chi jigt de ti-.i TS DH, CD M6n Vat li 121 d h a i d a u t u d i e n 1^ u va cifcfng dong d i e n qua no l a i He thijfc lien h e giiJa c a c d a i lu"crng l a : -2 "> -2 Giai V i... o n g day Gihi Goi N i , N2 l a so v o n g day ban dau cua m i cuon; n l a so vong day p h a i cuon t h e m T a c6: N N +24 ^ = 0,43; = 0,45 N, N, ^' N2 +24 +n N , = 120 0; N , = 516 ' ' = ^ n