BI 4 H LC PHNG

MC TIU BI GING

Pht biu c khi nim h lc phngHiu c cch thu gn h lc phng v tm thu gnHiu c iu kin cn bng tng qut Vit c cc dng iu kin cn bng ca h lc phngGii c bi ton tnh hc i vi h lc phng

NI DUNG BI HC:Phn I: Khi nim h lc phngPhn II: iu kin cn bng ca h lc phngPhn III: ng dng gii bi ton tnh hc v h lc phng

I Khi nim h lc phngQuan st mt s cc hnh v sau

Khi nim: H lc phng l h lc gm tp hp cc lc cng nm trong mt mt phng

II- iu kin cn bng ca h lc phng1- nh l di lc song songnh l: Tc dng ca lc F vo vt (S) ti im A s khng b thay i nu ta di lc F song song n im B v thm vo im y mt ngu lc c momen m ca lc F i vi im A

2 Thu gn h lc v tm thu gnXt vt kho st (S) chu tc dng ca h lc phng Yu cu: Thu gn h lc trn v tm O bt kQuy tc: p dng quy tc di lc song song i vi ln lt cc lc trong h lc phng

Kt qu:Ch : Hp lc ca h lc phng khng phi l vc t t do khng ph thuc vo tm thu gn ph thuc vo v tr ca tm thu gn O, nu thay i tm thu gn th m cng thay i

nh l: H lc phng bt k tng ng vi mt lc v mt ngu lc t ti mt im ty cng nm trong mt phng tc dng ca h lc. Chng c gi l lc v ngu lc thu gn. Lc thu gn t ti tm thu gn c vc t lc bng vc t lc chnh ca h lc, cn ngu lc thu gn c momen bng momen chnh ca h lc i vi tm thu gn. = 0

3- iu kin cn bng ca h lc phngiu kin tng qutnh l: iu kin cn v h lc phng cn bng l vc t chnh v momen i s chnh ca h lc i vi tm thu gn bng 0

b. Cc dng phng trnh cn bng ca h lc phngDng 1iu kin cn v h lc phng cn bng l tng hnh chiu cc lc ln hai trc ta v tng momen i s ca cc lc i vi im 0 bt k u bng 0

Dng 2iu kin cn v h lc phng cn bng l tng momen i s ca cc lc i vi hai im A v B bng 0 v tng hnh chiu ca cc lc ln trc Ox khng vung gc vi AB cng bng O

iu kin cn v h lc phng cn bng l tng momen i s ca cc lc i vi hai im A v B bng 0 v tng hnh chiu ca cc lc ln trc Ox khng vung gc vi AB cng bng 0 Dng 3

III - ng dng gii bi ton tnh hc i vi h lc phng Bi tonCho dm AB chu tc dng ca ngoi lc P = 60N nh hnh v. Hy xc nh phn lc lin kt ti hai gi A v B

Bi giiBc 1: Chn vt kho st l dm ABBc 2: Gii phng lin kt Bc 3: Lp iu kin cn bngBc 4: Gii h phng trnh v kim tra li kt qu

H phng trnh iu kin cn bng

Cng c kin thcKhi nim h lc phngCch thit lp cc phng trnh cn bng ca h phngng dng vo gii cc bi ton tnh hc v h lc phng

Hng dn t hcTrnh by cc dng phng trnh cn bngn li cc bc gii bi ton tnh hc v gii li v d va hc

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