3 2 -
SO 3-2011Datjl'Hpc
PhUdng phap sir dung chien liftfc de giai bai tap Vat li of THCS
(Phan giai bai tap cliuyen dong deu)
LE THI KIM LOAN Trudng THCS Nguyen TrUdng Tp - Dong Da - Ha Npi
I - Md BAI
Giai toan ve vat li la mpt khau quan trpng khi hpe bp mdn Vat li. Ben cgnh nhQng bai toan djnh tinh giup cho hpe sinh nlm vQng va eung eb kie'n thQc da hpe cdn can mpt he thbng bai tap djnh IQpng de giup hpe sinh cd the van dyng trong cupc sbng sau nay. Khi giai bai tap, dQ ehi giai cac bai toan giao khoa ve vat li thi bat ki mpt hpe sinh nao cung cd the gap nhieu hoac it khd khan trong khi van dung tri thQc da hoc. TrQ mpt sb khdng nhieu bai toan luyen tap ddn gian de ghi nhd cong thQc bieu dien mpt dinh luat vat li vdi yeu cau chu yeu la thay the cac trj so cua cac dgi lUdng trong cdng thQc va tim dai IQdng chUa bie't, edn dbi vdi cac bai toan vat li phai van dung nhieu djnh luat vat li (thudng gpi la cac bai toan tong hpp) hpe sinh ludn ludn cam thay khd khan. Da sb hpe sinh thQdng mle phai sai lam ngay khi vQa dpc de bai cho ring la da thay ro con dQdng giai bai toan; hole cho rang chi cd mpt con dQdng de giai bai toan; hoac la khong the thay dbi cho bai toan de lam eho nd trd thanh ddn gian hdn... Viee giup cho hpe sinh djnh hudng va giai quye't dUdc eae bai toan vat li mpt each logic va chinh xac la trach nhiem cua giao vien bp mdn vat li.
De hudng Qng phong trao viet sang kien kinh nghiem do Cdng doan nganh giao dye phat dpng, va de giup cho cdng viee giang day, hoe tap eua hpe sinh cd hieu qua chung tdi da nghien cQu, tham khao mpt sb tai lieu va qua thuc te giang dgy, roi phan tich tong ket va dUa ra chie'n lUdc giai bai tap vat li d trudng pho thdng cd sd vdi myc dieh giup cho hpe sinh ehu dpng, tieh cQc, sang tgo trong qua trinh nhan thue va de bie'n qua trinh nhan thUe cua hoc sinh thanh qua trinh tQ nhan thQc.
Trong khudn kho cua mpt de tai sang kie'n kinh nghiem, chung tdi se trinh bay gdi gpn trong phan toan chuyen ddng, day la loai toan hpe sinh da kha quen thude tQ bai tap toan tQ tieu hpe song hpe sinh van chi quen tu duy cung nhic theo kieu toan hpe ma chua cd dUpc cai
linh hogt va tinh tddng dbi cua bp mon vat li.
Sd dT ehung tdi chpn de tai nay vi sau khi ap dyng vap qua trinh giang day chung tdi thay qua viec tie'p can vdi phUdng phap giai bai tap nay, hpe sinh da chu ddng hdn, tu tin hdn va dl dung hudng hdn khi giai cac bai tap vat li.
TQ dd tao cho hpe sinh sQ say me, tieh cQc chu dpng trong hpe tap. Chung tbi viet de tai nay vdi mong muon nd se dQdc gop y, bo sung de trd thanh nhQng kinh nghiem giQp eho cbng viec dgy va hoc bp mdn Vat li ngay mpt tbt hdn. Do thdi gian cd hgn nen de tai chle con cd nhieu khiem khuyet, ehung tdi tin rang theo thdi gian, de tai eua chung tdi se dUdc bb sung, chinh li de ngay mpt hoan chinh hOn.
II - NOI DUNG
1. Trong qua trinh giang day bp mdn Vat li tdi bat gap da sb cae em hpe sinh khi giai bai tap djnh lUpng cua mdn Vat li da khbng dat cho minh mpt ke hogch cy the, ma cae em thudng dpc qua de bai roi chpn cac cdng thQc va tinh toan ngay, ne'u ehua ra ket qua thi cac em thudng mat phQdng hudng va de mle sai lam. Vi vay giao vien chung ta can hudng dan cho cae em hpe sinh nhOng phQPng phap giai tQng dgng bai cu the de hpe sinh cd dUdc nhQng dudng loi chung cho mpt dang toan ma d day chung tdi gpi la "chien lUde giai cac bai toan djnh IQdng ve mdn Vat li".
Xu the chung cua day hoc hien nay, ngudi ta coi trpng viee day eho hpe sinh Chie'n lUdc giai toan, nd khdng nhQng hQu ich doi vdi viec giai cac bai toan giao khoa, ma cdn can thiet hinh thanh eho hpe sinh mpt phong each khoa hoe tie'p can bai toan ndi chung, mpt dieu vd cQng quan trpng dd'i vdi hogt dpng lao dpng tuong lai eua cae em. Hoe nlm vQng v# van dung kheo leo cae chien lupc giai toan Vat II ngay tQ luc khdi dau viec giai toan se giup khic phue dUdc nhQng sai lam tren, nay sinh mdt mat do tam li cua ngQdi mdi hpe bp mdn khi dQng trudc mpt bai
DauvaHpc
* / \ ( . ; . \ i .\.ii
so 3-2011
33
toan Vat li thudng chtj y uu tien den viec tim ra Idi giai dung cua bai toan, va mat khac cdn chQa hieu rang giai toan la qua trinh tie'p can van de dQdc to chUe de thye hifn vi^c thu thap va XL/ li thdng tin theo tQng giai dogn.
Gigi toan muon dgt ket qua chic chin phai la mpt hanh dpng dupe to chUc mpt each cd ke hoach chat che va cd hieu qua nhlm dgt dQdc mpt myc dich xae djnh.
Chien luoc giai toan gbm cd 6 giai doan nhu sau:
1. Didn dat thanh ldi bai toan va tin chic bgn cd the giai dUdc nd.
2. Dinh rd tinh chat cua bai toan, tQc la phan tich thong tin da cung cap va xac djnh cai gi da biet va cai gi can bie't de giai dUdc bai toan.
3. Kham pha, TQc la dpng nap tim cac chie'n lupc tb chUc thdng tin da cho va tim cho dupc cai can biet.
4. Ke hoach, tUc la quyet djnh chpn mpt chie'n lupe hoac mpt nhdm chie'n lupc va lap cac bUdc hoac cac budc phy cho chie'n lUpc da chpn.
5. ThQc thi ke hogch
6. Danh gia, tUc la khlng djnh dieu da lam dudc, khang dinh da giai xong bai toan va tai sao giai dupe hoac tgi sao khbng giai dUde.
Trong khubn kho cCia de tai chung tbi xin ehi de cap den chie'n ludc giai toan chuyen dpng deu d trudng THCS
2. Logi toan ve chuyen dpng deu d THCS tuy khdng cd gi phUe tgp, nhUng la mpt trudng hdp rieng cua mpt logi toan dien hinh trong Vat li hpe la toan ve ddng hpe chat diem se hpe d Idp 10, ma den lue dd edn cd the ghep chung vao Idp bai toan bao quat hdn la toan ddng lye hpe. Do dd, lam quen vdi van de chie'n lUde giai toan djnh lupng ve Vat li bit dau tU loai toan nay la rat hpp li va can thie't.
Chle han ehung ta cd the giai dupe hau he't cae bai tap ve chuyen dpng deu da cho trong SGK Vat li Idp 8 (cai each), bdi vi cac bai tap djnh lUdng nay chi xoay quanh viee sQ dyng cdng thQc dinh nghTa cua van toe trong chuyen dpng deu v = ^ va cdng thUe tinh dudng di trong chuyen dpng ay s = v.t . DQ chung ta da giai dupe cac bai toan ve chuyen dbng deu ma khong gap khd khan nao dang ke thi chung ta eung van can lam quen vdi mpt con dQdng tong quat giai toan Vat li: dd la Chien lupc giai toan cy the vdi loai toan ve chuyen dpng deu gbm nhflng gpi y quan trpng giup chung ta djnh hudng dUdc suy nghT de sQ dung tbt nhat eae tri thQc da hoe ve chuyen dpng deu vao vie giai toan, ke ea nhQng bai toan phQc tgp hdn thupc logi nay. Dd la nhflng gdi y sau day:
a) Quan trpng nhat la phai chpn trudc vat mbc lam diem goc de tinh dudng dl trong mpt ehuyen dpng, rbi chpn chieu dUdng eho chuyen dpng. Thudng thudng chung ta chpn diem gbc va chieu dUPng sao cho thuan tien dbi vdi vip lam toan, va thudng de nhat la chon diem goc la diem ma vat d dd vao thdi diem t = 0. DT nhien eung ed khi chpn diem gbc sao eho thdi diem t = 0 vat cd s bing mpt gia trj nao dd thi lgi de dang lam toan hon, nhu
chung ta se tha'y dudi day.
b) Sau khi da chpn chieu dUdng tren trye toa dp (thudng la chieu tQ trai sang phai) thi chieu dUdng cua van toe cung dUde xae dinh. Dogn dudng di dQdc s se la dUdng neu d thdi diem t vat min d ben phai diem gbc va la am neu vat am 6 ben trai diem gbc.
c) Trong chuyen ddng deu, cac bai toan thudng yeu S
cau van dung cac cdng thUc v = ~ " va s = v.t.
Ddi khi do each chpn diem gbc ma chung ta ed the phai xet de'n dai lupng s^,, tUc la khoang caech eua vgt den diem gbc vao luc b i t dau khao sat chuyen dpng.
Mpt sb bai toan khd thupc chUPng trinh Vat li Idp 8 cd the can den khai niem van td'c tUdng dbi nhUng se dQdc li giai ddn gian nhQ mpt phep cpng dai sb cac van td'c.
d) De quen dan vdi viec giai toan Vat li, dau tien nen neu lai bai toan dQdi dgng eau hdi (ngdn ngfl ddi thQdng) rbi sau dd diin ta eau hdi dd sang dang ki hieu va phQdng ttrinh (ngon ngQ khoa hpe). Vi dy, neu eau hdi "Khi nao thi de'n mpt diem nao dd?" rbi dien ta thanh "Vdi gia trj t bing bao nhieu thi vat dat gia trj dQdng di s da eho?".
Hoac cau hdi "Vat d dau khi nd dat van tbc nao dd hige vao thdi diem nao dd?" va dien ta thanh "Dogn dUdng di s CO gia trj bang bao nhieu d thdi diem t da cho, Qng vdl van td'c v da eho?".
e) Cubi cQng hay ehu y tim hieu y nghTa cua dap so thu dQpc, Vi du, vj tri eua vat so vdi diem gbc, dp Idn cua dai lUdng can tim cd thich dang hay khdng...
f) NhQng gdi y tren day edn ed ich trong nhQng bai toan ve chuyen dpng se hpe d Idp mudi.
Sau day chung tdi sQ dyng phQdng phap vQa gidi thieu de phan tich ap dyng, giai mpt sb bai toan chuyen ddng ey the.
Vi dy 1
Mpt ngQdi di xe dgp xuat phat tQ diem O va chuyen ddng theo dQdng thing den diem M vdi van tbc deu 5m/
s. Sau 1 gid, ngQdi dd da d cah xa diem O mpt khoang each bing bao nhieu, va khi nao thi ngQdi ay de'n dQdc diem M? Biet ring OM = 36km.
* PhQdng phap giai
Ta chpn vat mbc thich hpp de khao sat ehuyen dpng eua xe dgp la mdt vat nao dd gin vdi Trai Dat (vf du diem O), dbng thdi nhd rang sQ lien he phu thupc giQa quang dudng di s, van tbc v va thdi gian t trong chuyen dpng thing deu la s = v.t. Cung phai chu y de'n cac ddn vi do, vi du met (m), giay (s) va met tren giay (m/s); hay kildmet (km), gid (h) va kildmet tren gib (km/h).
** Cac budc tien hanh eu the:
Chpn diem mbc la diem 0 tren dQdng chuyen dpng Qng vdi luc xe dap b i t dau xua't phat. va chpn chieu dQdng la chieu tQ O den M (xem hinh ve 1)
34.
SO 3-2011 _ _ . " J / \ i : . l l A.11DOUVMQC
"' '-'^- •y,} "rj;! iJll V L"
o = Ih M
(Diem goc)
Sau 1 gid ngQdi di xe dgp d dau?
Dien ta thanh kl hieu la : Vdi van tbc v = 5m/s, vat di dupe doan dQdng s bang bao nhieu sau khoang thdi gian t = Ih = 3600s?
Ta de dang tim dUde s qua cdng thQc dudng di s = v.t:
s= v.t = 5m/s . 3600s = 18000m = 18km.
Khi nao vat de'n dUde diem M, vdi OM = 36km?
Dien ta thanh ki hieu la: Thdi gian t bang bao nhieu ne'u dogn dudng s la
36km = 36000m? TQ cong thQc s =v.t ta tim dQdc:
5 36000w
t = - = - - ; — - — = 7200s = 2h
V 5m/ s
Bai toan nay kha ddn gian khdng can phai khao sat gi them.
Vi dy 2
Ta dung Igi bai toan d vi dy 1 de hieu kT hdn ve viee
) 0 r
chuyen diem gbc va chieu dUdng tren quy dao.
Giai:
* Trong chie'n lUdc giai toan thupc loai chuyen dpng deu ta cd the ndi ring viec chpn diem gbc va chieu dUPng cbt sao eho thuan tien cho vieclam toan. Trong Idi giai tren, ta da chpn theo each ddn gian va d i nha't: diem gbcc la diem O cua vat goc (mat dat) ma vat 6 do vao luc xuat phat (Qng vdi t = 0). Bay gid ta Xhii chpn each khac.
Chpn M la diem gbc, va chieu dQPng van la chieu tQ O de'n M. NhU the, khi vat bit dau chuyen dbng, nd da nlm tai vj tri each diem gbc M mpt doan dUdng bang MO = - 36km. Sd dT phai viet da'u trQ vi ta chpn trQde chieu dQdng la chieu tU O de'n M, va khoang each Mo thi hudng theo chieu ngupc Igi. SU thay dbi each chpn diem gbc nay dupe phan anh vao cdng thflc tinh dudng dl cua chuyen dpng bing each them vao mpt giai trj s^
gpi la gia trj ban dau cua dUdng dl:
s = s, + v.t.-H s = s„-H v.t (s. = -36km). '"'' •'•"•
O
M
(Die'm goc)
Day la cdng thQc tong quat tinh dQdng dl cua chuyen dpng thing deu, vdi S(, cd gia tri tuy thupc vao each chpn diem gbc. Neu ta trd Igi chpn diem gbc la O thi luc bit dau xuat phat (t = 0) vat nam ngay tgi diem gbc do do 3^= 0, va ta trd lai cdng thQc s = v.t nhQ da thay d vi dy 1. Neu chpn diem gbc la M va chieu dUdng nhu eu thi vdi t = 0 ta cd s = s„ = - 36km. Gia sQ Igi chpn diem P la diem gbc (P va M dbi xQng nhau qua O), vdi P n l m ben trai O va gifl chieu dQdng tQ O den M, thi lai phai dung cdng thQc tbng quat s
= Sg + V.t, trong dd s^, = -H 36km.
** Bay gid ta sQ dung cong thQc tbng quat s = s^ + v.t de giai bai toan vdi each chpn diem gbc la M va chieu dUdng di tQ O de'n M.
- Sau 1 gid xe dgp d dau? Hay: s bing bao nhieu khi t
= 3600s?
s = s„ + v.t = -36000m + 5m/s. 3600s = - 18000m Dau trQ d quang dQdng di eua xe dgp sau 1 gid ehi ed nghTa la IQc dd xe dgp cdn nlm d phia ben trai diem gbc M chu chua di tdi dudc diem gbc.
- Khi nao vat de'n dUde M, tflc la de'n dUdc diem gbc?
Hay: Thdi gian t bing bao nhieu khi dogn dUdng s trd thanh bing 0?
s -(-36000w)
0: So + v.t ® t =
5m/ s
= 7200s = 2h
Ta thay viee chpn diem gbc (eung nhu chieu dUdng) khdng anh hudng den dap sb, nhUng ddi hoi phai li giai ro y nghTa vat li eua ke't qua tinh toan. 6 vi dy dang xet la y nghTa cua gia trj - 18000m va s = 0 khi xe dap di de'n diem gbc.
VI dy 3
Ta tiep tuc sQ dyng bai toan 6 vi dy 1, va thay dbi diem gbc de tim hieu van de chuyen dpng tUdng dbi.
Giai
* Bay gid ta chpn diem gbc la diem gin lien vdi chinh ngudi di xe dap. Ne'u xet tUdng dbi vdi diem gbc (vat mbc) gin lien vdi xe dgp thi xe dgp ludn ludn dQng yen.Vay lam nhu the nao de tra Idi dUpte 2 cau hoi trong de bai loan?
+ s„ = s„ + V.I = 36000 - 5t .Aidicm goc dung \cii) AM = .s, = + 36km
O (t= ih)
()
(1 = I h )
M
Ne'u diem goc (vat moc) gin lien vai xe dap thi diing la xe dap dting yen, nhung so vcfi diim goc thi moi die'm tren doan duong OM a mat dat lai cung chuySn dong deu voi cung van tb'c v theo chidu nguoc lai. Gia sir chon die'm gdc la diim A gin lien xai xe iing vai diem O tren mat dat ma tii do xe bit dau xuat phat, va vln giii chieu duang nhu cu (tu trai sang phai tren hinh ve), thi cac didm O hay didm M ciia mat dat ciing chuyen dong deu voi van toe v
= - 5m/s. Va diem M thi nlm each didm goc A mot doan bing AM = + 36km.
Bay gid cau hoi thii nhat ciia bai toan se la:
Didm M (hay diem O ciia mat dat cung the") a dau (xet tuang doi vai diem goc A coi la dung ydn) sau 1 gid tir liic didn ra chuyen dong? Hay nd'u didn ta thanh ki hidu thi: Doan dudng di s bing bao nhidu vdi thdi gian t = 1 gid? (Nhd ring phai diing cdng thirc tdng quat tfnh dudng di s == s„ -i- v.t do die'm gd'c A khdng chon triing vdi diem M lUc bit dau didn ra
chuyen ddng). Ta se cd s = So H- v.t =3600 3m + (- 5m/s) . 3600s = 18000m. Gia tri duong ciia :; chiing td diem M vln cdn nlm ben phai didm gdc A.
Cau hdi thii hai ciia bai toan bay gid se la: Khi nao diem M ddn gap didm gd'c A gin hen \'di xe dap? Hay: thdi gian t bing bao nhieu dd doan dudng s trd thanh bing 0? (s trong cdng thiic tdng quat).
Ta cd: 0 = s„ -i- v.t = 36000m = (- 5m/s).t Do dd t = 7200s = 2h
Cac ke't qua vln khdng thay ddi khi thay ddi diem gdc va xet nhu chuydn dong ciia mat dat tuong dddi vdi xe dap coi la dung ydn.
Ta hay sir dung them chidn luoc lap bang dd' trinh bay cac ke't qua tfnh toan trong 3 \f du vira khao sat d trdn, nhd dd hinh dung cu thd true quan han ve each nghien ciru chuydn ddng vdi cac didm gdc khac nhau.
Diem gdc
O(da't) M(dat)
A (xe dap)
^ ^ ^ ^ ^ T h a i gian t Quang ^ " ^ ^ ^
dudng s ^ ^ ^ ^ s = v.t = 5tp
s = s„ -1- v.t
= - 36000 + 5t
s = S,| -1- v.t
= 36000 - 5t
A
o
t , = 0
0 - 36000m
+ 36000m
-1-
t j ^ l h
= 3600s
+ 18000m - 18000m
-h 18000m
M t, = 2h
= 7200s
+ 36000m 0
0
t,= 3h
= 10800s
54000m + 18000m
- 18000m
Vidu 4
Mot cand chuyen ddng vdi van td'c v khi nudc lang. Neu nudc chay vdi van td'c v' (tuong dd'i vdi bd sdng) thi thdi gian de cand di doan dudng s nguoc chieu vdi ddng nudc la bao nhidu? TTidi gian di xudi ddng nudc chay ciia cano la bao nhieu?
Gidi
* Bai toan nay thudng dugc giai dua tren lap luan: Khi chay xuoi ddng, cand dugc nudc day nhanh them ndn van tdc ciia nd dugc cdng thdm van tdc nudc chay, cdn khi chay ngugc ddng thi cand bi ddng nudc daiy liu nen van td'c ciia nd phai trir di van tdc nudc chay. Cach giai nay mang tfnh chat sd hgc hon la vat li, \i da khdng lam ndi bat tfnh chat tuong dd'i cil chuydn ddng: cand chuyen dgng tuong dd'i vdi vat mdc thir nhat la nudc cung dang chuvdn ddng tuang dd'i vdi vat mdc thii hai la bd song. Vdi hieu bie't so bd ve tinh tuong ddi ciia chuydn ddng theo SGK ldp 8 chiing ta cd the' giai bai toan nay
Chgn gd'c dudng di gin lidn vdi bd sdng va ttriing vdi didm xua't phat ciia cand, cdn chieu duang la chieu chuyen ddng cua cand. Xet tuang ddi vdi bd thi cand chuydn ddng trong nudc lang nhanh hon khi cand chuye'n ddng ngugc ddng nudc chay va cham hon khi chay xudi ddng nudc chay. Nhu the cd nghia la, van tdc ciia cand so vdi bd la kdt qua ciia mdt phep cdng dai sd giiia van tdc eiia cand vdi nudc lang va van td'c ciia nudc so \'di bd. Cd the khlng dinh, trong chuydn dgng thing deu thi van td'c V ciia vat 1 xet tuang dd'i vdi vat 3 bang tdng dai sd ciia van td'c v ciia vat 1 xet tuang dd'i vdi vat 2 va van td'c v' ciia vat 2 tuang dd'i vdi vat 3: V = v -t- v'.
Tir quy tic cdng dai sd cac \'an td'c tuang dd'i ndi
trdn, ta cd thd dd dang tim ra van td'c ciia cand so voi
bd khi chay ngugc ddng la : V, = v = v' va khi chay
xudi ddng la Vj = v H- V', trongdd cac van tdc v va v'
deu lay gia tri tuyet dd'i. "Thdi gian dd cand di hdt
doan dudng s khi chay ngugc ddng \a xudi ddng
se la:
3B.
i£- SO 3-2011 Jtoi.iHpc
,, - ^ \
v - v
V - l - VVi du 5: Mdt ngudi di xe may dudi theo mdt ngudi di xe dap dang chay d phfa trudc minh theo ciing chidu trdn ciing mdt dudng thing. Hg cung xua't phat mdt luc tai hai diem each nhau 20km, vdi van tdc lan lugt la 60km/h va 20km/h. Hdi sau bao lau thi ngudi di xe may dudi kip ngudi di xe dap, va tai noi each diem xuat phat ciia xe may bao nhidu kildmet?
Gidi
M ».
* Day la mdt bai toan thudc loai quen thudc da hgc trong chuang trinh sd hgc idp 5 bac tie'u hgc. Ta cd the' diing nhu mdt bai toan vat li trdn ca sd nhan thirc ranh mach tfnh tuang dd'i ciia chuydn ddng.
Cd hai each giai, tuy thuoc vao each chgn didm gdc.
Cdch 1: Chgn diem gd'c la diem la die'm O trdn mat da't, ung vdi didm xuat phat ciia xe may, va chieu duang la chieu chuyen ddng ciia xe may cd van tdc v (cung la chieu chuydn ddng ciia van tdc v' ciia xe dap.
D D
Dudng di ciia hai xe tfnh tir gdc O se la:
- Vdi xe may Sj., = v.t (vi s„ = 0)
- Vdi xe dap s^ = SQ -i- v'.t (.s,, = -i-20km vi D d bdn phai gd'c O)
v.t = s,, -I- v .t
t =V = V
Cdch 2: Bay gid ta chgn gd'c ciia chuyen ddng la chfnh ngudi di xe dap. De cho ggn ta ggi ngudi di xe may la vat 1, mat da't la vat 2 va ngudi di xe dap la vat 3, nd cung la vat mdc xhgn lam diem gd'c.
Chieu duong vln chgn la chidu OD nhu cu. Bay gid
-I-
— M ^
Khi nao hai xe gap nhau? Hay vdi t bing bao nhidu thi SM = s^?
Ta dd dang tim ra t ttr phuang trinh:
20km ^ ^, ^^ ,
= 0,5h = 30ph
60km/h = 20km/h
dudng di cua xe may xet tuong dd'i vdi diem gd'c D se la: s = s,, -i- V.t , trong dd s„ = - 20km vi M d bdn trai diem gdc, va V dugc tfnh la tdng dai sd ciia van tdc V ciia xe may tuang dd'i vdi da't cdng vdi van tdc v' ciia da't tuong dd'i vdi xe dap chgn lam gd'c.
Xe dap chon D lam vat mdc O' O Sv, = s„
O' la vi trf tuang dd'i ciia didm xuat phat O CLia xe may liic nd gap xe dap
Ta cd : s = So-i-(v - v').t
Khi nao xe may gap xe dap, tiic la ddn diem gdc? Hay vdi t la bao nhidu thi dudng di s bing 0?
Ta dd dang tun duoc phuang trinh : So -i- (v - v').t
=0 Hay:
0,5h = 30ph.
-(-20km)
v - v 60km/h-20km/h
Khoang each tir diem xuat phat ciia xe may de'n noi gap cd the tfnh dugc dd dang.
Ndu hai xe chay lai gap nhau thi van td'c ciia xe may tuong dd'i vdi xe dap se la V = 60km/h - (- 20km/h) - 80km/h, vi van td'c tuang dd'i ciia xe dap bay gid(v') lai ciing chieu vdi van td'c ciia xe may tuang dd'i so vdi da't.
Ta lai diing chie'n lugc lap bang dd trinh bay cac kdt qua tfnh toan trong bai toan vira neu tren.
M
o
Gd'c O(dat)
d c D
Xe may Xe dap Xe may
v -1-
^
Doan dudng s (km)
s = v.t = 60t s = s„ -1- v.t = 20 s = s„ -1- (v - v').t
D v' Nai xe may di
Thdi gian t (h)
+ 20t
= -20 + (60 - 20).t
0
0 20 -20
loi kip xe dap
0,5
30 30
1
60 40 . .- -
s
L - - - • -}•^^T'Ti
D^vdJHipC s6^-2011 " 7 ' /
Lap bang cho trudng hgp hai xe chay lai gap nhau cung khong kho khan gi.
Sau day la bang cho trudng hgip hai xe chay lai gap nhau:
37
GocO
Gd'cD
Xe may Xe dap Xe may
M v -1-
n V, . >
.^ ^
~~" ~--~,.__Thdi gian t (h)
Doan dudng s (km)""" —....,_____^
s = v.t = 60t
s = So -H v'.t = 20 - 20t
S = S,, -H (V -H V').t
= -20 + (60+20).t
A v' D
>Joi gap nhai^
0
0 20 - 2 0
1 4 15 15 0
1 2 30
10 20
Sau nay ( ddn ldp 9) khi da hg ham sd bac nhat y = ax - b va each bieu didn dd thi thi ta cdn ed the sir dung chidn lugc ve dd thi de khao sat cae bai toan vd chuyen ddng mdt each cd hidu qua.
3. Chidn lugc giai bai tap tren chiing tdi thuc hidn d ldp 8As va ldp 8As, khi giai bai tap vd chuydn dgng. Qua thuc te giang day chiing tdi nhan thay nhiing hgc sinh sau khi nlm dugc phuang phap nay
chil ddng, tich cue hon va dd mle sai lam han trong vide giai cac bai tap ve chuydn ddng. Trong khi dd hge sinh d hai ldp SA^ va 8A,„ chiing toi Iam ddi chiing thi thay kdt qua hgc sinh d hai ldp 8A5 va ldp 8As vugt trgi hon hin, khdng nhung d tfnh chii dgng tich cue ma cac em d hai ldp nay cdn nlm cac kidn thiic CO ban chic chin hon nhieu.
Sau day chung tdi xin trinh biy kdt qua thd'ng kd.
BANG THONG Kfi KET QUA D l £ u TRA TRONG HOC SINH
Ldp 8A,
8A,
8A,
8A,o
Tong so HS 45
39
46
50
Bai so 1 2 1 2 1 2 1 2
SdHStrenTB(%) 95%
100%
97%
100%
83%
86%
68%
72%
Sd HS kha gidi (%) 68%
75%
70%
82%
55%
58%
50%
59%
Qua bang thd'ng kd chiing ta thay ty sd didm cua hai ldp ap dung phuang phap giai bai tap theo chidn lugc vugt xa han han hai ldp ddi chiing. Song trong thuc te cd mdt dieu nghich h la sd gid bai tap ciia chuang trinh Vat li ldp 8 rat ft ndn dd thirc hidn de tai chiine tdi phai sir dung thdm gid bai tap cho hgc hoac hoc them mdt
each tu nguyen. Day cung la van de dat ra cho
chiing tdi can nghidn ciru thdm dd cd the ap dung
phuang phap nay mpt each hgp li va dai tra. Vdi lan
dau lam sang kidn kinh nghidm vd llnh \uc nay ndn
chle chin cdn cd nhidu thieu sdt. song chiing toi
mong ring vdi thdi gian nd se dugc bo sung, chinh If
de ngay mdt hoan thien han.
