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DOffMH^sds iVIpt phiTtfng phap sir dung do thj de giai mpt so bai tap Vat li THPT ThS NGUYEN HOANG QUANG Trudng THPT DT NT TUdng DUdng 1 Tuong DUOng • Nghe An I DAT VAN DE Bai tap vat ly trong day hgc[.]
DOffMH^sds iVIpt phiTtfng phap sir dung thj de giai mpt so bai tap Vat li THPT ThS NGUYEN HOANG QUANG Trudng THPT DT NT TUdng DUdng - Tuong DUOng • Nghe An I DAT VAN DE Vi du: Bai tap vat ly day hgc Vat ly la phuong tien de + Dd thj tga - then gian eua mot chat diem ehuyen cung CO kien thirc, ren luyen ky nSng phan tieh, long dong cua vat: x = f(t) hpp, xCr li thdng tin va gop phan phat trien tu vat ly -H Od thj dien tich - thoi gian cua mdt ban tu: q = q(t) Viec giai bai tap vat ly, ddi hdi hpc sinh phai sir dung tu + 06 thi cuong ddng dien - thoi gian trpng mach tai hien ve ban chat hien tuang vat ly, tir su dung dien bien doi: i = i(t) cac dinh luat, cdng thilre vat ly chi phdi hien tupTig de Sij dung thj nhir the nao giai bai tap vat ly? giai quyet van de dat Tuy nhien, khdng phai tat ea a, Nhan dgng thi vat ly cac b&i lap vat ly deu ap dung du^c cac eong thde, cac + Thong qua dinh !ual vat ly de tim mdi quan he giUa djnh luat vat ly duai dang tudng minh de giai Vdi cac dai lugng vat ly L va bien so vat ly t bai lap nay, dai lu^ng vat ly (ham so) phu Ihupe vao bien s6 vat ly khdng theo quy luat cd san bdi cac dinh + Xac dinh hinh dang dd thi L(t) luat vat ly, nen vifc tim gia tri eua cdc dai luoTig vat ly + Dd thi vat ly L(t) xac dinh pham vi bien thien theo bien so vat l^ doi hoi hpc sinh phai biel van dung eua bien sd t linh hoat c^c dinh lual vai ly chi phoi dd lim ldi giai + Neu bien so / bien thien lien tue thi thdng thudng gia Bai viSl dS xuil mpt phuong phdp kj-idc phuc khd khan tri L(t} bien thien lien tue ddi vdi nhQng bai tgp la: "M^t phuang phap sir + Neu quan he giua L vai I la quan he khdng phu thupc thi thi L(t) la tia song song voi Ot dung (id thi ^S S"'' "'P^ ^^ *«' '?/* "('' ^y THPT" + Neu quan he giua L(t) vai t la quan he phu thupe ti 1? II NOl DUNG CUA DE TAI thuan dd thi la doan thang qua O Do thi v^t li D6 thj vat ly la dd thi bieu dien s^r phy thupe cua mpt -H Neu quan he giUa L(t) vdi t la quan he bac nhal thi dai lirgtig v^l ly vho bien sd vat li: L= L(t) dd thj L(() la dudng thang Trong dd L la dai lui^ng vat li ggi la ham sd, t la dai -t- Neu quan he giua L(t) vdi t la quan he bae hai thi luting vat li bien doi gpi la bien so thj L(t) la parabol b, MQt phirrrng phap van dung thj de giai bai tgp viu ly I 'Hiong thudng t bien ihien mpt dai luong J/ thi L bien thien mpt lupng AL AL c '\ trudng hgp viec tinh loan theo N cdng thuc Id don gian, nhien ta cd the m bieu dien quan h^ giQ- AL va At theo thj khid6AL=dt(MNPQ) {hinh I) -Neu AL C ma C Igi la mpt ham so ciia t ihi khdng the phan tich dan gian AL = CAi ma phai diing mgt hai M " M Al H'mh I i t s o 5-2012 B%?i«f phuong phap; la dung gia trj trung binh cua C (chi dung C phu thupc bac nhat vao L); each thlr hai la sij dung thi C(t) ( quan h? giQa C va t) thi L chinh la dien tich hinh gach cheo {hinh 2) Khi cac quang dudng bdng lien tiep chinh la dien tich cua cac hinh thang va tam giac lien tiep + Theo gia thiet: khoang thai gian vat het quang dudng giua la A/j = B,B^ = Is A 5 , j4, dong dang ABB, A, nen: BB, Hinh Vi du: Kxw tinh cong cua luc dan hdi Tuy nhien: De tinh dien tich cua hinh thang cong thi lien quan den viec tinh toan tich phan, vdi hpc sinh THPT khong ehuyen ihi khong the diing phuong phap dugc Do d6 cac vi du minh hoa se trinh bay de lai chi tap trung vao nhung vi du ma su phu thupc cua C vao / {C(t) la ham sd bae nhSt hoac dd thj la cac doan ihang) Vidu minh hga Vidu J Mdt vat ehuyen dpng cham dan d^u di dupe ba doan dudng bang lien tiep trudc dung Thdi gian vat di het quang dudng giua la ]s Tinh ldng thdi gian vat di het ba doan dudng bang ke tren bang phuang phap dd thi van tdc Nhan xet dl{BB,A^) _SBj_ dt(BB,A,) SB,+1 _1_ (1) =>SBj = V + l ( j ) Ai?S2.42d6ngdang tJiOA nen: OB J dl(BB,A,) ^ dt{BOA) ^3 OB = S.BBj = S.{42 BB^ _ ~dB~~S + \)is) ^gy- tdng thdi gian vat di het ba quang dudiig bang k^ tren la M = Vs (V2 -f- \){s) Vidu iVlot doan lau di tir ga den ga kc vdi tdc dp trung binh 54kni/h khoang thai gian 20 phiit Liic rdi ga nay, tau ehuyen dong nhanh dan deu thdi gian 2,5 phut, 15 phut tiep theo tau ehuyen ddng d6u va thdi gian cdn lai tau ehuyen dong cham dan deu ddn lue dirng lai Tinh gia tdc ciia doan tau Nhan xet Vdi bai nSu su du:ig cdng thiic tinh quang dudng va Ndu sir dung cdng thuc vd ehuyen ddng thing ddu va eong, thuc van Idc cung cho ket qua nhung rat phuc ,H^^g bigp ddi dfiu thi viec tinh loan kha dai ddng Sir tap da dong Su dyng dd Ih van tdc la phuong phap j ^ g dd thi van l6c se cho kdt qua nhanh va don gian hon don gian de hieu va ngan gpn hon nhiSu, day la lai thd ciia phuang phap thi , , , , ,1 A I- -• "*" '^'^i ^^^'^ ^^^ ^^u ehuyen dong nhanh dan deu khdng liuffiig dan gnu ; , , : van toe ban dau thoi gian 2,5 phut va giai + Chpn ch,cu duong la chieu ehuyen dpng, ta cp thj j „ , „ ^ j ; ,j„ ,|,„yj„ a5„g ^ | , j ^ ^j„ j j „ j „ ^^ ji,„g van toe - tho, g,an (l,i„h vl i) l^i ,^„„g ,^j, ^.^^ 35 phut nen gi,- t6c cua hai giai dpan cp dp Ion bang + Chpn chi^u duong la chigu ehuyen dpng ta c6 d6 thj van t6c- thai gian {hinh ve 4): Av(kmlpli) tVmh + Ta chia tam giac OAB tfianh ba phSn co di?n tich bang bdi cae doan thang song song vdi true Ov nhir hinh ve \ r '(!'•!!> ° 2.5 17.5 20 Hinh 31 IMfMHbpcs6s + Quang dudng tau di tir ga din ga kt l/a(s7m) 20 s = v.A/=54.—= 18fon 60 s = dt(OABQ 20+15 2.18 14,4 (20 + 15).2,5 35 20+15 ,0.2,5 (fo)i/;7A') = 0,114m/s^ Vay gia tpc ciia dpan tau h-ong hai giai doan dau va cupi CP cung dd Idn la 0,114ffi/r yi dif Mpt vat bai dau ehuyen dpng ma khong cd mpt thdi diem nap gia tpc eiia no bang khdng Dp thj phu thupc gia tdc vao van tdc ciia n6 nhir (/z//i/i ve 5) ^ ' A a(m/s-) I I H >v(m/s) " Hinli + Tir d6 thi tren ta th^y van tdc eua vat phu thupc thdi gian nhu sau: S 12 van tdc {m/s) thdi diem(5) , T- j - - j - u- - - u\ it • i r + Tu ta CP dp thi van toe- then gian nhu {hinh 7) dirdi day v(m/s) Hinh Tim khoang thai gian tir thdi diem ban dau 1=0 cho den luc van toe vai dat dupe 12m/s va quang dudng vat di dupe thdi gian Nhan xet Vdi bai tap Ihj nay, neu ehuyen cac thdng so tren thj ve gia irj cac dai lupng rdi ap dung cdng thuc da cd de linh loan chic chan se phirc tap han nhieu so vdi vi?c sir dung true tiep thi, dd phai ehuyen thi ve dgng sy phu thupc eua 1/a vao vgn toe cua chat diem v& Ihj ciia van tdc iheo thdi gian Giai + Tir cdng IhCrc: fl = Av/A/ => / a = A/ / Al' => Ar = Av, / a (*) dd ta dyng thj Ma theo v tjiinh ve 6) + TLT thj vua dung dupe, ket hpp vdi (*) la thay v0t dgl v^n tdc V = 12m/s sau mpt khoang thdi gian bang di^n lich ciia hinh OABCDEGH: A/= 1.4-1-0,5.4 + 0,25.4 = 75 + Ta cung cd the linh thdi gian theo cong thiic V = Vg + a.{t - /„) vi v(*fi moi giai dogn a la hSng so Quang dudng vat di duac lu liic bai dau ehuyen ddng den liic vat dat van tdc 12m/s la(4 + 8)-2 (8 + 12X1 s = di{OMNPQ) = = 30m 2 Vay thdi gian va quang dirdng ma vat di dupe cho ldi liic vgi dai duoc van tdc 12m/s lan lirpi la 7s va 30m III KET LUAN Trong qua Irinh giai bai lap vat ly hpc sinh thudng ap dung cac cdng thuc, eae dinh luat vai ly cd s5n hay su dung phuong phap phan tich, long hpp, , de xac dinh cae dai lupng lien quan Nhung gap phai mpt sd dang bai tap vat ly hpc sinh thudng lung ning qua trinh giai Vdi mdt sd bai tap vat ly da neu de tai thi viec ap dung thi de giai sS nhanh va hieu qua De lai da giiip lipc sinh giai quyel kha tdt cac bai toan ve dd ihi, trang bi cho hpc sinh phuong phap xac djnh cac mdi lien he giua cac cdng tliuc vat ly va cdng thirc hinh hoc, gdp phan phat tnen tu vat li ndirieng,W ve sy vai hi?n tupng cho hoc sinh Vi vay, de tai vua dam bao linli khoa hpc, tinh chinh xac, tinh sir pham, tinh sang tao va linh hieu qua TAI LIEU THAM KHAO Nguvin Danh Ba -Tuyiiii l^p cac bai lap vpl li nang cao NXB Nglif An, 2004 Nguvin Van Hanh- Ban iin vgi ly thong s6 • 1997 fliii Quang Hdn Giai toan vpi li 10 ijip I- NXB Giaoduc H 2009 VB TJwnh Khiei - Kicn ihuc CO ban va nang cao v^t li 10- NXB Giao due H 2005 Nguvin Thi Khoi - V j i If 11 nang cao (Sich gido vt£n)- NXB Giao due, H 2007 ... De tinh dien tich cua hinh thang cong thi lien quan den viec tinh toan tich phan, vdi hpc sinh THPT khong ehuyen ihi khong the diing phuong phap dugc Do d6 cac vi du minh hoa se trinh bay de