Tài liệu Thiết kế bài giảng vật lí 12 nâng cao tập 1 (nxb hà nội 2008) trần thúy hằng, 252 trang

TRAN THUY HANG HA DUY£N TUNG NANG CAO TAPMOT NHA XUAT BAN HA NOI ,,,i-'ft TRAN THUY H A N G « - H A DUYfiN TUNG •« THIET KE BAI GIANG cm T^rtlSMH ^ "iHt NANG CAO - TAP 1 NfiA XUAT BAN HA NOI .ai not ddu Thie't ke bai giang Vat li 12 nang cao dvroc viet theo chifdng trinh sach giao khoa m6i ban hanh nam 2006 - 2007. Sach gidi thif u m6t each thi§'t ke bai giang VSt li nang cao theo tinh than doi mdi phifdng phap day - hoc, nham phat huy tinh tich ciTc nhan thiic cua hoc sinh. Ve noi dung : Sach bam sat noi dung sach giao khoa VSt li 12 theo chifdng trinh nang cao. O moi tie't, sach chi ro muc ti§u ve ki^'n thiic, ki nSng, c»c c6ng viec chuan bi ciia giao vi^n va hoc sinh, cac phtTOng tien ho trd giang day can thie't, nham dam bao chat Iifdng tufng.bai, timg tie't len Idp. Ngoai ra sach co md rong, bo sung them m6t sd' ndi dung lien quan tdi bai hoc bang nhieu hoat dong nham cung cS'p them tu" lieu d6 cac th^y, co giao tham khao vSn dung tuy theo do'i ttfdng hoc sinh tiing dia phiTdng. Ve phifdng phap day hoc : Sach diTdc tri^n khai theo hvrdng tich cue hoa hoat ddng cua hoc sinh, lay cd sd cua moi hoat dong la nhiing viec lam cua hoc sinh dudi su" htrdng d i n , phvi hdp vdi dac trvmg mon hoc nhiT : thi nghif m, thao luan, thu'c hanh, nham phat huy tinh doc lap, tu" giac cua hoc sinh. Dac bi^t, sach rat chii trong khau thu'c hanh trong timg bai hoc, ddng thdi ciing chi ro tung hoat ddng cu thg cua giao vien va hoc sinh trong mdt tiS'n trinh day hoc, coi day la hai hoat ddng ciing nhau trong do ca hoc sinh va giao vign la chu the. Chiing tdi hi vong cud'n sach nay se la mdt cdng cu thie't thUc, gop p h i n hd trd cac thiy, cd giao giang day mdn Vat li 12 trong viec nang cao hif u qua bai giang cua minh. Cac tac gia CHi/ONG 1. DONG Ll/C HOC VAT RAN • • • • BAII CHUYEN D 6 N G QUAY CUA VAT RAN QUANH M 6 T TRUC c 6 DESfH I - MUC TifiU 1. V^" kien thtic - Hoc sinh hieu duoc khai niem vat rdn va chuyen ddng tinh tien cua vat ran la gi. - Hoc sinh hieu duoc khai niem toa dp goc (p. - Xay dung dupe cac cdng thiic tdc do goc co, gia tdc gdc y. - Nam viJng cac cdng thiic lien he giiJa tdc do gdc va tdc do dai, gia tdc gdc va gia tdc dai ciia mot diem tren vat rSn. - Xay dung dupe cac phuong trinh ddng hpc ciia chuyen ddng quay. - Van dung dupe eac cdng thiic ciia chuyen ddng quay deu, quay bie'n d6i diu de giai cac bai tap don gian. 2. Ve ki nang - Ren luyen cho hpc sinh kT nang sii dung phuang phap tuong tu de xay dung cac khai niem, cdng thiic cua chuyen ddng cua vat ran. II - CHUAN BI Giao vien - Ve hlnh 1.1, 1.2, 1.4, 1.6 trdn gia'y kho AQ. Hoc sinh - On tap lai phin ddng luc hoc chat diem b Vat li Idp 10 THPT. Ill - THifiT Kfi' HOAT D 6 N G DAY - HOC Hoat d o n g cua hoc sinh Tror giiip cua giao vien Hoat dong 1 Kiem tra, chuan bi dieu' kien xuat phat. Dat van de GV neu cau hdi kiem tra kie'n thiie cu. - Neu khai niem vat ran. - Chuyen ddng tinh tien la gi ? HS suy nghT ca nhan tim cau tra Idi. HS nhan thiic dupe van de cua bai hpc. - Viet phuong trinh chuyen ddng cua vat chuyen ddng thing bie'n doi deu. Viet cdng thiic van tdc cua vat dd. Dgt vdn de : 6 Idp 10, chiing ta da xac dinh quy luat chuyfin ddng ciia vat ran chuyen ddng thang bie'n doi deu. Bai hpc nay chiing ta di sau tim hieu va xac dinh quy luat chuyen ddng, tim mdi lien he giOa eac dai lupng dac trung cho chuyen ddng quay cua vat ran quanh true CO dinh. Hoat dong 2 Tim hieu khai niem toa do goc HS quan sat hinh ve 1.1 tren kho gia'y AQ, suy nghT ca nhan tim eau tra Idi. — Mdi diem tren vat vach mdt dudng tron nam trong mat phang vudng gdc vdi true quay, n kinh bang khoang each tir di6m do de'n true quay, tam ciia quy dao nam tren true quay. - Mpi diem ciia vat deu quay duoc cung mdt gdc trong cung mdt khoang thdi gian. GV neu cau hoi de HS tim hieu khai niem toa dp gdc: - Xet.mdt vat ran bat ki quay quanh mdi tmc Az t d dinh (Hinh 1.1 SGK). Hay cho bie't dac diem ciia quy dao cua mdt di6m tren vat ran khi vat ran chuyen ddng va gdc quay ciia cac diem tren vat ran? Tro g i u p ciia giao vien Hoat d o n g cua hoc sinh : GV thdng bao: HS tie'p thu, ghi nhd. ; - Quan sat tren hinh ve, vi tri cua vat tai mdi thdi diem se dupe xac dinh bang mdt gdc (p giira mat phang ddng P gin vdi vat va mdt mat phing cd dinh PQ. Gdc

0. Hoat dong 3 Xay dung khai niem toe do goc trung binh va tdc do goc tiic thdi GV neu cau hdi de HS xay dung khai niem tdc dp gdc: HS thao luan chung toan Idp. j - Khi vat rin quay, su bie'n thien theo thdi gian ciia (p the hien quy luat chuyen ddng quay cua vat rin. Hay tim mdt dai lupng dac trung cho miic dp quay nhanh hay cham eiia vat rin. Dai lupng dd dupe xac dinh nhu the nao ? GV neu cae cau hdi gpi y: - Dai lupng dac trung cho miic dp | quay nhanh hay cham dupe xac : - Trong chuyen ddng thing, dai lupng nao dac trung cho chuyen ddng nhanh hay cham ciia vat ? Dai lupng dd dupe xac dinh nhu the nao? dinh bang thuong so —^ • At ' : - Tuong tu, trong chuyen ddng quay, ta cd the tim mdt dai lupng dac trung cho miic dd quay nhanh hay cham ciia vat rin dupe khdng ? - 0 thdi diem t, toa dp gdc ciia vat la g). 0 thdi diem t +At, toa dp gdc ciia vat la Hoat dong ciia hoc sinh Tror giup cua giao vien (p + A(p, dai lupng dac trung cho miic dp quay nhanh hay cham ciia vat rin dupe xac dinh nhu the nao ? GV thdng bao: - Dd chinh la tdc dp gdc trung binh j C0f^J = cua vat rin trong khoang thdi At gian At. HS thao luan chung toan Idp. - Tdc dp gdc tiirc thdi d mdt thdi diem t duoc xac dinh bing gidi han - GV yeu cau HS xay dung cdng thiic xac dinh tdc dp gdc tiic thdi va cho biet don vi cua tdc dp gdc. ciia ti sd —^ khi A / tien dan tdi 0. At co = hm i^

0 At - Don vi ciia tdc dp gdc la rad/s. Hoat dong 4 Xay dung khai niem gia tdc goc trung binh va gia tdc goc tiic thoi GV neu cau hdi de HS xay dung khai niem gia tdc gdc: - Hay tim mdt dai lupng dac trung cho su bie'n thien nhanh hay cham ciia tdc dp gdc. Dai lupng dd dupe xac dinh nhu the nao? HS thao luan chung toan Idp. - Dai lupng dac trung cho su bie'n thien nhanh hay cham cua tdc dp gdc dupe xac dinh bing thuang so Aco At GV neu cac cau hdi gpi y - Trong chuyen ddng thing, dai lupng nao dac trung cho sir bie'n thien nhanh hay cham ciia van tdc ? Dai lupng dd dupe xac dinh nhu the nao ? - Tuang tu, trong chuyen ddng quay, ta ed the tim mdt dai lupng dac trung cho Trd giiip cua giao vien Hoat dong cua hoc sinh su bie'n thien nhanh hay cham ciia tdc dp gdc dupe khdng ? - O thdi diem t, vat cd tdc dp gdc la co. O thdi diem t + At, vat cd tdc dd gdc la (o + Ao). dai lupng dac trung cho sir bie'n thien nhanh hay cham cua tdc dd gdc dupe xac dinh nhu the nao? GV thdng bao: - Dd chinh la gia tdc gdc trung binh Ao) Yi^ = HS thao luan chung toan Idp - Gia tdc gdc tire thdi d mdt thdi diem t dupe xac dinh bing gidi han , , . Aft) , , . » cua tl so ..-J.- , , ,, , ... cua vat ran trong khoang thai At gian At. - GV yeu cau HS xay dung cdng thiic xac dinh gia tdc gdc tiic thdi va cho bie't don vi cua gia tdc gdc. ,. r, khi A r tien dan tOi 0. At 7 = Hm hay y = coit) ,v->o At -) - Dan vi cua gia tdc gdc la rad/s . Gia tdc gdc tiic thdi ciia vat rin quay quanh mdt true d thdi diem t la dai lupng dac trung cho su bie'n thien ciia tdc dp gdc d thdi diem dd va dupe xac dinh bang gdi han , , ^, Ao) ., . . .., ,.- ,. _ cua tl so khi A t tien dan toi 0. At Hoat ddng 5 Xay dung cac phuong trinh dong hoc ciia chuyen ddng quay GV yeu cau HS phat bilu khai niem gia tdc gdc. GV neu cau hdi de HS xay dung phuang trinh chuyen ddng ciia vat rin quay deu: Hoat dong cua h9c sinh Trg giup cua giao vi^n - Trong trudng hpp tdc dd gdc ciia vat rin khdng doi theo thdi gian thi chuyen ddng ciia vat rin la chuyen ddng quay d6u. Toa dp gdc ciia vat d thdi diem / dupe xac dinh nhu the nao ? HS thao luan chung toan Idp. GV neu cac cau hdi gpi y. - Hay neu su tuang ling giOa cac dai lupng gdc trong chuyen ddng quay va cae dai lupng dai trong chuyen ddng thing? - Viet phuong trinh chuyen ddng ciia vat chuyen ddng thing deu. Tuong tu rin quay diu la : (p = (pQ + cot, trong nhu vay chiing ta cd the vie't dupe dd q)Q la toa dp ban dau luc f = 0. phuang trinh chuydn ddng cho vat chuyfin ddng quay diu nhu the nao ? Phuong trinh chuyen ddng ciia vat GV neu cau hdi de HS xay dung cac cdng thiic trong chuyen ddng quay bie'n doi deu: - Trong trudng hpp gia tdc gdc ciia vat rin khdng doi theo thdi gian thi chuyen ddng ciia vat rin la chuyen ddng quay bie'n doi deu. Hay vie't cdng thiic xac dinh tdc dp gdc ciia vat d thdi diem t, phuang trinh chuyen ddng eiia vat rin, cdng thiic ddc lap vdi thdi gian trong chuyen ddng quay. HS thao luan chung toan Idp. - Cdng thiic xac dinh tdc dp gdc ciia vat chuyen ddng trdn bie'n doi deu tai thdi diem / la : co = coQ + yt 10 GV neu cac cau hdi gpi y: - Viet cdng thiic xac dinh van tdc ciia vat chuyen ddng thing bie'n doi deu tai thdi diem t, phuong trinh chuyen ddng eiia vat va cdng thiic lien he giira van tdc, gia tdc va dp ddi. Hoat dong cua hoc-sinh Trd giup cua giao vien - Phuang trinh chuyen ddng quay bie'n ddi diu : - Tuong tu cac cdng thurc cua chuyen ddng thing bie'n ddi deu, hay vie't cac cdng thiic cua chuyen ddng quay. 1 2

0). - Neu tdc dp gdc giam dan theo thdi gian thi chuyen ddng quay la cham din ir<0). Hoat dong 6 Tim hieu cac dac diem cua van toe va gia toe ciia cac diem tren vat quay GV neu cau hdi de HS tim hiiu cac dac diem ciia van tdc va gia tdc cua cac diem tren vat quay. - Ne'u vat rin quay khdng deu thi gia tdc cua mdt diem tren vat rin dupe xac dinh nhu the nao ? HS thao luan chung toan Idp. GV neu cac cau hdi gpi y: - Ne'u vat rin quay d6u thi gia tdc cua mdt diem tren vat rin dupe xac dinh: - Ne'u vat rin quay deu, gia tdc ciia mdt diem tren vat rin dupe xac dinh nhu the nao ? Cd the su dung cdng thiic gia tdc hudng tam da hpc d Idp 10 de xac dinh gia tdc eiia mdt diem tren vat rin dupe khdng ? v' 2 . . , a„- — = 0} r, gia toe a^ dac trung r cho su thay doi ve hudng eia van tdc dai V. 11 Hoat dong ciia hoc sinh Trg giup cua giao vien 1 : : - Neu vat rin quay khdng deu thi ; mdi diem ciia vat rin cung chuyen \ ddng trdn khdng deu. Khi dd vecto : van tdc v ciia mdi diem thay doi : ea ve hudng lin dp Idn. Khi dd, ; vecto gia tdc ciia mdi dilm cd hai thanh phan : Thanh phan Z^ cd phuong vudng • - Khi dd, gia tdc cua mdt dilm tren vat 1 rin dac trung cho tinh chat gi ciia van , tdc dai V ciia nd ? - Neu vat rin quay khdng deu thi vecto j van tdc dai v cd dac diem gi ? i - Cd thi sii dung hai thanh phin gia tdc dl dac trung cho sir thay ddi ve hudng va ve dp Idn cua vecto van tdc dai v dupe khdng ? Ne'u dupe, thi hai thanh phan cua vecto gia tdc dupe xac dinh nhu the nao ? gdc vdi V, dac trung cho sir thay ; doi ve hudng ciia v, thanh phan • nay chi'nh la gia tdc hudng tam. ; Thanh phan a, cd phuang ciia v, \ dac trung cho sir thay doi ve dp Idn • ciia V Av V - VA ; a, = — = At At Aco rco - rcOf) = - =r = ry Al At : I • ; - Dp Idn cua vecto gia tdc toan ; - Vecto gia tdc toan phin duoc xac dinh phan dupe xac dinh ; ; thdng qua hai thanh phan vecto gia tdc ; hudng tam va tiep tuye'n nhu the nao ? / 2 2 a = ^a„+a, : - Hudng ciia vecto gia tdc toan | phan duac xac dinh vecto a hpp ; vdi ban kinh OM gdc a, vdi i tana = - ^ = ^ - a, Y • «« CO ; 12 ; i Trg g i u p ciia giao vien Hoat d o n g cua hoc sinh 1 1 Hoat dong 7 Cling cd bai hpc va djnh hudng nhiem vu hoc tap tiep theo GV yeu cau HS lam viec vdi phieu hpc ; tap; - Yeu cau HS dn tap lai eac kie'n thii'c ; vl: mdmen lire, phuong trinh ddng lire ; hpc ciia chat dilm, y nghTa cua khdi ;lupng. PHIEU HOC TAP Cau 1. Mdt canh quat dai 20cm, quay vdi tdc dp gdc khdng doi la 94 rad/s. Tdc dp dai cua mdt dilm d vanh canh quat bing A. 37,6 m/s. B. 23,5m/s. C.l8,Sm/s. D. 47m/s. Cau 2. Banh da cua mdt ddng co tir liic khdi ddng de'n liic dat tdc dp gdc 140rad/s phai mat 2s. Bie't ddng ca quay nhanh dan diu. Gdc quay ciia banh da trong thdi gian tren bing A. 140 rad. B. 70 rad. C. 35 rad. D. 35 n rad. Cau 3. Mdt banh xe quay nhanh din diu quanh mdt true. Liic t = 0 banh xe cd tdc dp gdc 5 rad/s. Sau 5s tdc dp gdc cua nd tang len de'n 7 rad/s. Gia tdc gdc cua banh xe la A. 0,2rad/s' B. 0,4rad/sl C. 2,4rad/s^ D. 0,8rad/s^ Cau 4. Hai dilm M,, M2 tren mdt dTa CD, khoang each tir tam cua dTa den hai dilm lan lupt la 2 /?,, 7?2 v^ R\=-R2Cho dTa CD quay deu quanh mdt true vudng gdc vdi tam ciia dTa. a) Ke't luan nao sau day diing khi ndi vl gdc quay ciia hai dilm trong cung mot khoang thdi gian ? 13 A . (Pi =^i = —(y2- 2 B. &>2 = —2=9ft)|. D. 0)2=0}^. c) Kit luan nao sau day diing khi ndi vl tdc dp dai cua hai dilm ? 2 A. V, =-V2. 2 B. V 2 - - V , . C. V2=2v|. D. V2=3v]. BAI2 PHUONG TRINH D O N G L U C HOC CUA VAT R A N QUAY QUANH M 6 T TRUC c6 DEsJH I - MUC TifiU 1. Ve kie'n thiic - Xay dung dupe bilu thiic bilu diln md'i lien he giira gia tdc gdc va mdmen lire ciia vat rin cd true quay cd dinh. - Dua ra eac du doan vl su phu thudc eiia mdmen quan tinh va thiet ke dupe phuang an thi nghiem kilm tra du doan. - Nim dupe khai niem mdmen quan tinh dd'i vdi mdt true cua vat rin. - Nim va van dung dupe phuang trinh ddng luc hpc cua vat rin quay quanh mdt true dl giai dupe mdt sd bai tap. 2. Ve ki n a n g - Van dung kie'n thiic vl mdmen luc da hpc d Idp 10 dl xay dung bilu thiic bilu diln mdi lien he giira mdmen luc tac dung vao vat rin va gia tdc gdc. 14 - Ren luyen kT nang dua ra cac du doan cd can cii. - Ren luyen kT nang thiet ke cac phuang an thi nghiem kilm tra du doan, thu thap sd lieu, xu li sd lieu va dua ra ket luan. - Van dung phuang trinh ddng luc hpc ciia vat rin quay quanh mdt true de giai mdt sd bai tap. II - CHUAN BI Giao vien - Bd thi nghiem khao sat chuyin ddng tinh tien - chuyin ddng quay ciia he vat. - Ve bang 2.1 vao giay kho AQ. Hgc sinh - On tap lai kie'n thiie vl mdmen luc, phuang trinh ddng luc hpc ciia chat dilm, y nghTa ciia khd'i lupng. III - THlfiT Kfi' HOAT DONG DAY - HOC Hoat dgng cua hgc sinh Trg giiip cua giao vien Hoat dong 1 Kiem tra, chuan bi dieu kien xuait phat. Dat van de HS suy nghT ca nhan tim cau tra Idi. - Tir bilu thire tinh mdmen cua lire : M - F.d, suy ra tac dung lam quay canh cira ti le vdi tich dp Idn ciia luc tac dung va canh tay ddn cua nd. - Bilu thiic ciia dinh luat II Niuton _ F a- — m HS nhan thiic dupe van dl cua bai hpc. GV neu cau hdi kilm tra kien thiic cu: - Tac dung lam quay canh ciia phu thudc vao yeu td nao ? Bilu thiic toan hpc nao bilu thi dilu dd ? - Viet bilu thiic ciia dinh luat II Niu-ton. Dgt vdn de : Trong chuyin ddng ciia cha't dilm, giiia gia tdc cua chit dilm va lire tie dung cd mdi lien he dupe diln ta bing dinh luat II Niu-ton. Trong chuyin ddng quay cua vat rin, gia tdc gdc va mdmen luc cd mdi lien he vdi nhau nhu the nao ? 15 Hoat d g n g cua hgc sinh Trg giiip cua giao vien Hoat ddng 2 Tim mdi lien he giua gia tdc gdc va mdmen luc HS suy nghT ca nhan, sau dd thao luan chung toan Idp. GV yeu cau HS tim bilu thiic toan hpc bilu diln mdi lien he giiia mdmen lire va gia tdc gdc trong chuyin ddng quay eiia vat rin. GV neu cac cau hdi gpi y: O- ^ - Xet trudng hop don gian nha't la vat rin gdm mdt qua cau nhd khd'i lupng m gin vao mdt dau thanh rat nhe, dai /•. Vat chi chuyin ddng quay tren mat phing nhin nim ngang xung quanh mdt true thing diing di qua dau O ciia thanh. Khi dd bilu thiic lien he giira mdmen lire va gia tdc gdc dupe viet nhu the nao ? T, - Luc tac dung theo phuang nao thi gay gia tdc tie'p tuye'n a, cho vat ? Luc F, gay ra gia tdc tie'p tuyen - Mdmen ciia luc F, dd'i vdi true quay cho vat: F, = ma,. O dupc xac dinh nhu the nao ? Mdmen ciia luc F, dd'i vdi true quay O : M - F,r = majr =i> M = I wr jY - Mdmen tac dung len mdi chat dilm dupe xac dinh M, = I m^r, 1 Y - Vl cac cha't dilm ciia vat rin cd cung mdt gia tdc gdc nen mdmen tac dung len vat rin dupc xac dinh : ( \ I / / M = Y,M,= I 16 - Gia tdc tie'p tuye'n a, cd lien he vdi gia tdc gdc Y r^hu the nao ? Bilu thiic bilu diln mdi lien he giiia gia tdc gdc ciia vat rin va mdmen luc dupc vilt nhu the nao ? - Ne'u vat rin gom nhilu chat dilm khdi lucmg Wp m,.. a each true quay nhirng khoang each i\, /j,... khac nhau thi mdmen tac dung len mdi chat diem dupc vilt nhu the nao ? Mdmen tic dung len vat rin dupc xac dinh nhu the nao ? Trg giiip cua giao vien Hoat d g n g cua hgc sinh Hoat dong 3 Xay dung khai niem mdmen [ quan tinh, vie't phuong trinh ; ddng luc hgc ciia vat ran quay ; quanh mot true cd djnh GV neu cau hdi vl van de cin nghien ciiu: - Dai lupng ^ w ^ r , trong bilu thiic / lien he giira mdmen ciia vat rin vdi gia tdc gdc dac trung cho tinh chat gi ciia vat rin ? Dai lupng dd phu thudc vao nhiing yeu td nao ? HS thao luan chung toan lap va j • dua ra du doan. - Dai lupng ^ ^ , 7 - , dac trung ; / '• cho miic quan tinh cua vat quay. | GV neu cac cau hdi gpi y: - T i r b i l u thiic lien he giira mdmen ciia vat rin vdi gia tdc gdc, vdi ciing mdmen tie dung, vat rin nao cd dai lupng Z ' " , ' ' , Idn thi gia tdc gdc cd HS dua ra cac du doan : Du dodn I : Dai lupng ^ m,r, phu | gia tri nhu the nao ?,Cd d l dang thay doi van tdc gdc cua vat rin hay khdng ? thudc vao mdmen luc tie dung ; len vat rin va gia tdc gdc ma nd i \^ 2 M g a y r a : 2^m,r, = I r Dudodn 2 : Dai lupng ^'W/A', chi ; / 1 phu thudc vao khd'i lupng ciia vat • rin va su phan bd khdi lupng ciia • vat rin. i HS thao luan chung toan Idp dl • thiet ke phuang an thi nghiem. GV yeu cau HS thie't ke phuong an thi nghiem dl kilm tra cac du doan tren. - Can phai cd mdt vat rin, tac ; dung vao vat rin va do mdmen luc ; • dd, do gia tdc gdc ciia vat rin. GV gidi thieu bd thi nghiem va gpi y. 17 Hoat d g n g cua hgc sinh Trg giiip cua giao vi§n - Tac dung luc vao vat ran bing cac qua gia trpng, luc tac dung vao vat rin bing luc cang T. - Mdmen luc tac dung vao vat rin: M = T.R. Can phai xac dinh dp Idn luc T va ban kinh R. Luc T = m^ig - (3|), vdi m,, a, la khdi lupng va gia tdc ciia qua gia trpng. - Gia tdc ciia qua gia trpng dupe 2sx xac dinh: a^ ="V' Vay mdmen tx luc tac dung vao dTa trdn cd thi xac dinh : - Chiing ta cd bd thi nghiem: gom cac vat rin la nhiing dTa trdn (1, 2, 3) cd the quay quanh true di qua tam dTa. Cac dia trdn gidng nhu cac rdng rpc. Tac dung lire vao vat rin bing each nao ? - Xac dinh mdmen luc tac dung vao vat rin nhu the nao ? - Tren bd thi nghiem cd bd tri 2 cong quang dien va ddng hd do thdi gian, cd the xac dinh dupe gia tdc ciia qua gia trpng dupc khdng ? M=T.R=m\(g-a\)R =m,(8-^-)R - Gia tdc gdc cua vat dupc xdc dinh thdng qua gia tdc tie'p tuye'n, mat khac gia tdc tie'p - Gia tdc gdc ciia vat rin dupe xac dinh nhu the nao ? - Gia tdc gdc ed quan he vdi gia tdc tie'p tuye'n nhu the nao ? Hoat dgng cua hoc sinh Trg giiip ciia giao vien tuye'n chinh bing gia tdc cua gia —• trong. Vl vay Y = —^ = R R,t,' - Tien hanh thi nghiem vdi luc tac dung la T,, xac dinh dai - Can phai tien hanh thi nghiem nhu the nao dl kilm tra du doan 1. lupng y^mjrj tuong irng. Sau dd tiln hanh thi nghiem vdi luc tac dung 7^2, xac dinh dai lupng ^W,A; tuang ling. Tir dd riit ra ke't luan. HS chii y quan sat, ghi sd lieu, xir li sd lieu va riit ra ke't luan. GV tien hanh thi nghiem dl kilm tra du doan thii nha't ciia HS. Ke't ludn : Vdi mdt vat rin cd true quay nhit dinh, thay doi mdmen luc tac dung vao vat rin thi gia tdc gdc thay doi nhung ^/W/A", ludn la mdt hing sd. Vay dir doan 1 la khdng chinh xac. GV neu cau hdi vl thi nghiem can phai tiln hanh dl kiem tra du doan 2. - Can phai tien hanh thi nghiem vdi hai dTa trdn cd cung chat cau tao, cung khd'i luang nhung ban kinh khac nhau (su phan bd khdi lupng khac nhau). Xac dinh dai - De kilm tra du doan 2 can phai tien lupng Z»i,r,^ (viec xac dinh dai /?3 = 5cm. hanh thi nghiem nhu the nao ? Biet ring eac dTa trdn cd khd'i lupng va ban kinh lan lupt nhu sau : W] = 800g, R^ = 5cm; W2 = 800g, R2 = 2,5cm; m^ = 400g, 19 Hoat dgng cua hgc sinh lupng ^m^ri Trg giiip cua giao vien gidng nhu thi nghiem vira tien hanh d ben tren). Tir dd riit, HS ra ket luan. - Can phai tien hanh thi nghiem vdi hai dTa trdn cd cung ban kinh nhung khdi lupng khac nhau. Xac dinh dai lupng ^ w / , . . Tit dd, HS riit ra kit luan. HS chii y quan sat, ghi sd lieu, xir li sd lieu va riit ra kei luan. GV tien hanh thi nghiem. Kei ludn : Dai lupng ^ OT,r, khdng chi phu thudc vao khdi lupng eiia vat rin ma cdn phu thudc vao sir phan bd khdi lupng cua vat rin. HS tiep thu, ghi nhd. GV thdng bao: - Mdmen quan tinh / ddi vdi mdt true la dai luong dac trung cho miic quan tinh cua vat rin trong chuyin ddng quay quanh true ay. i - Dp Idn ciia mdmen quan tinh cua vat rin khdng chi phu thudc vao khd'i lupng ciia vat rin ma cdn phu thudc vao su phan bd khd'i lupng ciia vat rin. - vay ta cd thi vie't: M - l.y, phuang trinh nay gpi la phuang trinh ddng luc hpc ciia vat rin quay quanh mdt true. 20 Trg giiip cua giao vien Hoat dgng ciia hgc sinh Day la phuang trinh co ban ciia chuyin ddng quay ciia vat rin. Hoat dong 4 Van dung phuong trinh co ban ; ciia chuyen ddng quay ciia vat : rin de lam bai tap GV yeu ciu HS lam viec vdi phieu hpc •• tap dl lam bai tap. 1 HS suy nghT ca nhan, sau dd thao ; luan nhdm va dai dien nhdm len ; bao cao kit qua. ; Ap dung dinh luat II Niu-ton cho ; chuyin ddng tinh tiln cua thiing ; nude, ta cd : ; mg-T = ma (1) Ap dung phuang trinh ddng luc • hpc cho chuyin ddng quay cua vat ; hinh tru, ta cd : ; M = TR= l.Y (2) : GV neu cac cau hdi gpi y: - Viet phuang trinh dinh luat 11 cho thiing nude. - Viet phuang trinh ddng lire hpc cho vat rin la hlnh tru cd true quay. - Mdmen quan tinh dupc xac dinh nhu the nao ? - Gia tdc gdc dupc xac dinh thdng qua gia tdc tie'p tuye'n nhu the nao ? Mat khac, ta lai cd : y = —, thay • R • . (2) ,ox va- rut '. ra : Trr = -^ ly = -—• la • '• : vao : R R^ j Thay T vao (1) ta dupc : ma la - ma ; R^ mg m+ 1 1 -^ R- , 1 \+ ^ r- mR^ 1 \ • ; Hoat dong 5 Cling cd bai hgc va dinh hudng : nhiem vu hoc tap tie'p theo - GV yeu eau HS lam viec vdi bang 2.1 SGK dl tim sir tuong tu giira hai phuang 21