Giai Birdrc V 20 A = — = — = 0,5m = 50cm f 40 song So bung song t r e n day l a : N = ^ = = 0,5X, 25 T r e n day c6 bung song va n u t song Vay chgn dap an A Cdu 20: M o t mach dao dpng dien tii 11 tiTorng dang c6 dao dong dien tCf t u T a i khoang nufa thori thdi gia t r i diem = 0, dien t i c h t r e n mot b a n t u d i e n t cifc d a i Sau gian ngSn n h a t A t t h i dien tich t r e n ban t u bang m o t d a i Chu k i dao dong r i e n g cua mach dao dpng l a : cxXc A A t B A t C A t D 12At Giai Vi d i e n t i c h t = t h i q = Q o nen phiTPng t r i n h dao d p n g cua dien t i c h la: q = QoCOSO)t = T a i At t h i q = ^ ^ ^ = Qo ^TT T a CO c o s — A t = - T QoCOSYt — T cos — A t TT At = -