Thy Minh Sn - TTGS - LTH Tr Tuhttp://www.yeuvatly.org CON LC
TRNG PHNG I. KIN THC C BN 1. S trng phng. - Xt 2 con lc dao ng
trong 2 mt phng song song, con lc 1 c chu k T1, con lc hai c chu k
T2 (T1>T2). Ti thi gian t = 0 hai conlc c cng 1 trng thi (VD:
cng qua VTCB theo chiu + chng hn), sau thi gian no trng thi ca 2
con lc li ging nh trng thi lc t = 0 (tc li cng quaVTCB theo chiu+)
c gil s trng phng. Thi gian* nh nht k t khi thi im t = 0 cho ti lc
2 cn lc trng phng ln th nht gi l chu k trng phng. 2. Phng php gii.
2.1. Phng php 1 Gitl thi gian trng phng ca 2 con lc k t lc t = 0.
-S dao ng m con lc 1 thc hin c l: 11tNT= (1) -S dao ng m con lc 2
thc hin c l: 22tNT= (2) (v trng thi c lp li nh ban u nn s dao ng l
nguyn tc 1 2, NN Z e ) T (1) v (2) 1 21 1 2 22 1T NN T N TT N = =
(3) S cp s 1 2( , ) NNtha mn (3) s tng ng vi s dao ng m 2 con lc
thc hin c cho ti khi trng phng. Con lc trng phng ln u tin ng vi cp
s 1 2( , ) NNl nh nht, khi th 1 2( , ) NNv (1) hoc (2) s tm c thi
gian trng phng ln u.
Ch:Thigiantrngphnglnuktlct=0cngchnhlchuktrngphng A hay min 1min 1
2min 2. . t N T N T A= = = . 2.2. Phng php 2 V con lc 2 c chu k nh
hn con lc 1 nn sau ln dao ng th nht ca con lc 2 con lc 1 cn 1 thi
gian (T1-T2) tr v v tr xut pht ban u ca n. Ni cch khc l con lc 1 b
tr so vi con lc 2 l (T1-T2).Sau n ln dao ng ca conlc 2 khong thi
gian tr nysl 1 2*( ) n T T . s trng phng xy ra th khong thi gian tr
trn phi ng bng mt chu k T2.Hay: 1 2 2 1 2*( ) ( 1) n T T T nT n T =
= + = A(4) (thi gian trng phng ln th nht). T trn ta c th tnh c chu
k trng phng: 1 21 2. T TT T A=(5) Ch : Cng thc (5) ch c th p dng
khi 22 1TT T e (***) hayne, Nu tnh ngc (ngh l cho , T1 yu cu tnh T2
chnghn) thsau khi tnh ra ps phikim tra (***) c tha mn cha. II. BI
TP Bi 1: Hai con lc n c chu k T1 = 1,6s, chu k T2 = 1,8s dao ng nh
trong hai mt phng song song, hai con lc cng qua v tr cn bng theo
cng chiu lc t = 0. Xc nh thi im gn nht m 2 con lc lp li trng thi nh
trn. Gii Cch 1: Gi t l thi gian m 2 con lc lp li trng thi ban u.
Trong thi gian t s dao ng m 2 con lc thc hin c ln lt l N1 v N2. Thy
Minh Sn - TTGS - LTH Tr Tuhttp://www.yeuvatly.org 11 11 2 22 2 1
1229;18; 27...1, 68;16; 24... 1,8tNT NT N Nt N T N NNT== = = ==(v
N1, N2 nguyn) Thi im ln u gp nhau ng vi N1, N2 nh nht. Hay N1 = 9,
N2 = 8.Vy, thi gian gp nhau ln u: min 1 1min 2 2min. . 1, 6.9 14, 4
t T N T N = = = =(s) Cch 2: Sau ln dao ng th nht ca con lc 1, con
lc 2 cn 1 khong thi gian (T2-T1) ch v v tr c ca n. Tc, con lc 2 tr
hn vi con lc 1 mt khong thi gian 1,8-1,6 = 0,2 (s). Sau n dao ng ca
con lc 1, khong thi gian tr ca con lc 2 s l:*0, 2 n . trng thi 2
con lc lp li nh ban u hay s trng phng xy ra th thi gian tr ny phi
bng ng chu k T1. Vy: 111, 6*0, 2 80, 2 0, 2Tn T n = = = =(dao ng)
Hay thi gian 2 con lc trng phng l: 1*8 14, 4 T =(s) Bi 2: Con lc th
nht c chu k T1 = 3s, con lc th 2 c chu k T2. Hai con lc trng phng
ln th nht cch nhau 99s. Tnh chu k T2. Gii: Haiconlc trng phngln
thnht cchnhau 100s tc chu k trng phngl 100s. theo cng thc (5) ta c:
121. 99.3 99( )99 3 34TT sTA= = =A + + (s) Bi 3: Hai con lc n treo
cnh nhau c chu k dao ng nh l T1=4s v T2=4,8s. Ko 2 con lc lch mt gc
nh nh nhau ri ng thi bung nh. Hi sau thi gian ngn nht bao nhiu th
hai con lc ng thi tr v v tr ny? A. 8,8sB. 12sC. 6,24sD. 24s Hng dn:
tnh chu k trng phng A p n D. Bi 4: Tip theo bi ton 3, hi hai con lc
trng phng ln th 5 sau bao nhiu pht v 2 con lc thc hin c bao nhiu
dao ng A. 1 pht, 20 v 22 dA. 1 pht, 25 v 35 dA. 2 pht, 25 v 30 dD.
Kt qu khc
