Hinh 9.1
H I Vat nao tac dung vao day cung lam eung bie'n dang ? Vat nao tac dung vao mui ten lam mui ten bay di (Hinh 9.1) ?
I - LUC. CAN BANG LUC
0 Trung hgc ca sd ta da hgc luc va can bdng lire.
Vdi khdi nidm gia tdc d chuomg trdn, ta cd the dua ra dinh nghia vd luc va cdc luc can bdng nhu sau :
Hinh 9.2. Duang thing AB mang vecta luc F goi la gia cCta Iqc F.
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Hmh 9.3
f-S^ Ve cac lUc can bang tac dung len qua eau (Hinh 9.3). Cae lUc nay do nhung vat nao gay ra ?
1. I uc la dai lucmg vecto ddc trung cho lac dung cua vat nay len va! khae md ket qua Id gay ra gia toe cho vai hodc lain cho vat hien dqng. 9 1
2. Cdc luc can hdng Id cdc luc kid tdc dung ddng ttud vao mot vol tlu khong gay ro gia tdc chu vqt.
3 . Dudng thdng mang vecta luc ggi la gid cua lire (Hinh 9.2). Hai luc can bang la hai luc ciing tac dung ldn mdt vat, eiing gia. ciing do ldn va ngugc chidu. S
Dan vi ciia luc la niutan (N).
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il - T 6 N G HOF^ LUC
Trong toan hgc, mud'n tim vecto C la tdng ciia hai vecta .4 va fl (C = ^ -h fl)ta phai dp dung quy tdc hinh binh hanh. Dd la tfnh chat can ban ciia cac dai lugng vecta. Vay khi ta ndi luc Id mdt dai lugng vecto thi nd cd tfnh chat nay khdng (Hinh 9.4) ?
Ai
T2
> Tau keo H;r/i J.-t
1. Thi nghiem
a) Ta bd tri mdt thf nghidm nhu d Hinh 9.5 trdn mdt tdm bang dat thdng dumg. Vdng nhdn O (coi nhu cha't didm) dimg ydn dudi tdc dung cua ba luc F|, Fj va F3 (cd do ldn bdng trgng lugng ciia ba nhdm qua can).
N
2 y'
Liia
Hinh 9.5
b) Ve trdn bang ba vecta bidu didn ba luc dd (chgn tl xfch la 1 dom vi do dai iimg vdi trgng lugng ciia mdt qua can). Vecto OA bidu didn lire F^, vecta OB bidu didn luc F, va vecta OC bieu didn luc ^^3.
Vi hai luc F, va F2 can bdng vdi luc F^ ndn mudn cho vdng nhdn vdn diimg ydn thi luc thay the chiing phai la mdt vecta F (dugc bidu didn bang vecta OD) cd do ldn F = F^va ngugc hudng vdi vecta F3. Ta nhan thay tii gidc OADB la mdt hinh binh hanh (d day Id hinh chir nhat) vdi OA va OB Id hai
canh, cdn OD la dudng cheo (Hinh 9.6). Hinh 9.6
c) Thay ddi do ldn va hudng ciia cdc luc F, va F^, g g jU thf nghiem tren ta rut ra thi khi vdng nhdn dumg ydn ta vdn cd nhan xet nhu dugc ke't luan gi ve tinh chat the. Si ^ ^ 3 '"^^^ •^
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F2
Hinh 9.7 Tong hop hai Idc dong quy.
2. Dinh nghla
Tong hop luc Id thay the cdc luc tac dung ddng then vao cung mcU vat hdng mot luc cd tac dung i'lom; het nhu cac luc ay.
Luc thay thd' nay ggi la hop luc.
3. Quy tac hmh binh hanh
_ _ . Sell hai luc ddng qu\ lam thanh hai canh cua mdt
K : Trong trudng hop cd nhieu . i i i i , .i ' J . , i . i- A .,_- - .i tmh hmh hanh. till duirni! cheo ke tu ciiem dong lue dong quy thi van dung quy , , ciT,r.h a i\
t i c nay nhuthe nao ? '""' ''"'" ''"'" '"'" ''"' ''"" '''"'"' ( " • " " ^•^^•
Vd mat toan hgc, ta vie't : F - F\ -\- F2.
11: . oiFi I i^fPN CAN BANG CUA CHAT DIEM Vdi khdi nidm hgp luc, ta cd th^ phat bidu didu kidn can bdng cua chat diem nhu sau :
Muon cho mcjt chdt diem dung can bang thi hop life cua cac luc tde dung len nd phdi bdng khdng.
F = F, + F2 + ... = 0
IV - PHAN TICH LUC
Fi N
Fo O
f^ 1. Ta cd thd giai thfch sir can^bdng cua vdng nhdn O theo mdt cdch khdc. Luc F3 trong thf nghiem d Hinh 9.5 cd hai tac dung. Mdt mat nd keo day 1 _ theo hudmg MO, mat khae nd keo da^ 2 theo hudng
^2 NO. Do dd, ta cd thd' thay thd' luc F3 bdng hai lUc F,' va F2' theo hai phuang MO va NO. Hai luc nay Hinh 9.8 F\ can bdng vdi hai luc F^ va F2 (Hinh 9.8).
2. Djnh nghla
Than tich luc la tliay the mot luc hdng hai hay nhii'u luc CO tdc dung giong hei nhu lue du.
Cdc luc thay the nay ggi la cac luc thdnh phdn.
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M N
F\
3. Mud'n phan tfch luc F, thdnh hai luc thanh phdn ^ F|' va F2' theo hai phuang MO vd NO, ta lam nhu ^ sau : Tir ddu miit C cua vecta F3 ta ke hai dudng ^^2
thdng song song vdi hai phuomg dd, chiing cdt G p nhiimg phuomg nay tai cac didm E va G. Cac vecta I ^
OE va OG bidu didn cdc luc thdnh phan F,' va F,' C H/nAj 9.9 (Hinh 9.9).
4 C>^" "
Phan tfch luc Id phep lam ngugc lai vdi tdng hgp luc, do dd nd ciing tuan theo quy tdc hinh binh hanh. Tuy nhidn, chi khi biei mat luc co tdc dung cu the theo hai phuang ndo thi mdi phdn tich luc do theo hai phuang dy.
Luc la dai luong vecto dac trung cho tac dung ciia vat nay vao vat khae ma ket qua la gay ra gia tdc cho vat hoac lam cho vat bien dang.
Duong thdng mang vecto lire goi la gia cua luc.
Oon vi cua luc la niuton (N).
Tong hop luc ia thay the cac lut tac dung ddng thdi vao cimg mdt vat bang mo irc co tac dung gidng het nhu cac luc ay. Luc thay the nay goi la hop luc.
Quy tdc hinh binh hanh : Neu hai lire ddng quy lam thanh hai canh cua mdt hinh binh hanh, thi dudng cheo ke t u d i e m ddng quy bieu dien hop lire cua chung.
Dieu kien can bang ciia mdt chat diem la hop luc cua cac luc tac dung len no phai bang k h d n g :
F = F^ + F2 + • •• = 0.
Phan tich luc la thay the mdt lire bang hai hay nhieu luc ed tac dung gidng het nhu luc do.
Phan tich mdt luc thanh hai lut thanh phan dong quy phai tuan theo quy tac hinh binh hanh.
Chi khi biet mot lut cd tac dung cu the theo hai phuong nao thi mdi phan tich lut theo hai phuong ay.
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