K h i quay v d i toe n t h i cam k h a n g cua cuon day l a : Z -2Z Z,, 2Z, ^ Vgy chgn dap an C Cdu 31: O m a t t h o a n g cua mot chat l o n g c6 h a i nguon song k e t hop A va B each 20 cm, dao dong theo phiTcfng thSng dufng vdri phiTcfng t r i n h U A = 2eos40 7rt va U B = 2eos(407tt + T I ) ( U A va U B t i n h bSng m m , t t i n h bang s) B i e t toe t r u y e n song t r e n m a t chat long la 30 cm/s Xet h i n h vuong A M N B thuoc m a t thoang chat long So diem dao dong vdri b i e n ciic d a i t r e n doan B M l a : A 19 B 18 C 17 D 20 Giai Theo de b a i t a c6: - T a n so f = — = = 20Hz 2n 2% v 30 - Birdrc song K = — = — = 1,5cm f 20 X e t t r e n doan A B , dao dong cua B n h a n h pha h o n A la TI nen v a n t r u n g tarn O l a m o t ciic tieu 10 So d i e m CLfc d a i t r e n doan OB eo t a t ca N , = = 13,33 0,75 Ta chi chon p h a n nguyen n e n N i = 13, ufng vdri 13 v a n giao thoa se cat M B , tao t h a a h t r e n M B ciic d a i Cac v a n t r e n doan O A chi c6 N2 cSt M B , t a t i m so N2 - H i e u difdrng d i tCf M den A , B l a : Ad = M B - M A = 20V2 - 20 « 8,28 cm - H a i nguon dao dong ngu:oc pha nen dieu k i e n M la mot Ad = Nhir vay, N2 = CLTC dai: X = 8,28 « k + - = - ^ = 5,5 1,5 CLTC dai T o n g so circ d a i t r e n M B la: N = N i + Ng = 13 + = 19 Vay chgn dap an A 70 Giii chi tiSt de thi TS DH, CD Mon VSt I