, d ^ J Uu
Chuang IV TC/TRUCING
21.4. Gia sfl hai đng difn /i va I2 chay trong
hai day đn vudng gdc vdi mat phang hinh ve, ngugc ehidu nhau nhu ffinh 21. IG.
Tai M : Vectd cam ting tfl fli do /i gay ra ed gde d M, ed phucmg vudng gde vdi CM,
cd chidu nhu tren ffiinh 21.1G ; vectd cam
flng tfl fl2 do I2 gay ra cd gd'c d M, ed phucmg
vudng gdc vdi DM, cd chidu nhu tren Hinh 2ỊIG. Vi CMD = 60° (tam gidc CMD đu, eanh a) nen gdc gifla hai vectd fl, va fl2 la 120°.
Mat khae, hai veeto fl, vd fl2 cung đ dai :
= 10-^ T
fl, = fl-, = 2.10"^.^ = 2.10""^ ^ -"^'^
a 10-1
Vectd cam flng tfl tdng hgp fl = fli + Bj la dudng cheo hinh binh hanh vdi hai canh la fli vd fl2. ffinh binh hdnh nay lai la hinh thoi (vi Bi = fl2)
do đ fl ndm tren dudng phan giac cua gdc (fli,fl2) ngMa la fl ± CD (ffinh
21.IG).
Bdi vi gdc (fl,fli) = (fl,B2) = 60° nen tam giac tao bdi 5,5, hoac fl2,fl Id đu, do đ :
B = fl, =fl2= 10"^T
21.5*. Ta chgn mat phdng hinh ve vudng gdc vdi hai đng difn /i va I2 : ggi Oi
va O2 la giao didm cua hai đng difn vdi mat phang dy (Hinh 21.2G).
1. a) Vi M each /i : 6 cm, each /2 : 4 cm ma 6 + 4 = 10 cm = O1O2 "^^^ ^
phai ndm tren doan 0,02- N Ddng II gay ra tai M vecto cam -^ X X 1
flng tfl flj ed phuomg vudng gdc p
vdi OiM, cd đ ldn : "
Ddng I2 gay ra tai M vectd cam flng tfl fl2 cd phucmg vudng gde vdi
G2M, ed đ Idm :
Bl 2.10-l^r?:T = 4,5.10"^ T "0,04
Ca hai vecto fl, va fl2 đu cung phuong, eung chidu nen : fl = flj+fl2 = 6,5.10"^T
b) Vi N0i=6 cm, NO2 = 8 em vd NOi + NO2 = O1O2 ntn tam giac
NO1O2 vudng gde tai Ậ
Ddng 11 gay ra tai N veeto cam ting tfl flj ed phUdng vudng gdc vdi NOi nghia la ndm theo NO2 va ed đ ldn :
—»
Ddng I2 gay ra tai Á' vecto tfl cam fl2 cd phucmg vudng gdc vdi NO2
nghla la ndm theo ÂOi va cd đ ldm :
fl, = 2.10-''.7r^ = 2,25.10-^ T Vi fli 1 fl2 nen :
0,08
fl = ijflf + fl| » 3.10-^ T
7 7 - • - » - » - » - »
2. Ta phai tim diem P de cho tai đ fl, + fl2 = 0 nghia la fl, va B2 eung
phucmg, nguge ehidu va eung đ ldm.
Didu kifn fli vd fl2 cflng phuong budc P phai ndm tren dudng thdng Oi02- Didu kifn fli vd §2 ngugc chidu bude P phai ndm ngoai doan O1O2 (vdi didm M e O1O2, hai veeto fli va B2 cflng phucmg, cflng chidu).
Dd ldn eua hai vectd dy fli = 2 . 1 0 - 1 - ^ , flj = 2.10"^.-^^ phai bdng nhau:
J C/2
nghia la
Dd dang suy ra : FOi = 20 em ; PO2 = 30 cm.
Trong mat phang vudng gdc hai đng difn, didm P vdi POi = 20 cm,
PO2 = 30 em la didm tai đ B = 6
Trong khdng gian, quy tfch eua P la dudng thang song song vdi hai đng difn, each /i : 20 em, each I2 : 30 em.
130 Bl = POi PO2 " 1 " B2 => h POi 6 " 9 " F O i ' _ h PO2 2 '' 3