43 Heä löïc ñoàng quy F ok caùc löïc ñi qua O F kx 0 0 F k F ky 0 211 F kz 0 Do

43 heä löïc ñoàng quy f ok caùc löïc ñi qua

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4.3. Heä löïc ñoàng quy ( F ok ), caùc löïc ñi qua O: F kx = 0 0 ( F k ) F ky = 0 (2.11) F kz = 0 Do M 0 = m 0 ( F k ) = 0 töï thoûa maõn. 4.4. Heä löïc phaúng ( F k ), F k Oxy: Vôùi ñieåm A tuøy yù thuoäc maët phaúng löïc Oxy: m A ( F k ) = m Az ( F ). k = d k F k k . Vector moment cuûa caùc löïc naøy ñeàu cuøng phöông neân ta coù theå quy öôùc thay theá m A ( F k ) bôûi giaù trò ñaïi soá: m A ( F k ) = m Az ( F k ) = d k F k (2.12) Ta laáy daáu (+) hoaëc (-) theo quy taéc moment cuûa löïc ñoái vôùi truïc (hình 2.7). k j O z y m A ( F k ) A z y (2.9) 0 ( F k )
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Baøi giaûng moân Cô lyù thuyeát Hoïc kyø 2, naêm hoïc 2012-2013 17 Ñieàu kieän caân baèng cuûa heä löïc phaúng coù 3 daïng: 4.4.1 Daïng 1: F kx = 0 0 ( F k ) F ky = 0 (2.13) m A ( F k ) = M A = 0 vôùi A tuøy yù thuoäc maët phaúng löïc. Ñieàu naøy hieån nhieân do (2.12) 0 M 0 R A ' 4.4.2 Daïng 2: F kx = 0 0 ( F k ) M A = m A ( F k ) = 0 (2.14) M B = m B ( F k ) = 0 (vôùi Ox AB ) 4.4.3 Daïng 3: M A = m A ( F k ) = 0 0 ( F k ) M B = m B ( F k ) = 0 (2.15) M C = m C ( F k ) = 0 Söû duïng phöông phaùp chöùng minh treân: neáu R A 0 thì R ’phaûi coù giaù chöùa ñoaïn AB vaø AC, do A,B,C khoâng thaúng haøng neân khoâng theå xaûy ra tröôøng hôïp treân. 5. BAØI TOAÙN CAÂN BAÈNG CUÛA VAÄT RAÉN: 5.1. Moâ hình baøi toaùn:
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Baøi giaûng moân Cô lyù thuyeát Hoïc kyø 2, naêm hoïc 2012-2013 18 Moät vaät raén khoâng töï do (chòu lieân keát) chòu taùc duïng cuûa löïc (löïc hoaït ñoäng) ñang caân baèng. Nhöõng yeâu caàu ñöôïc ñaët ra laø: Xaùc ñònh caùc phaûn löïc lieân keát. Tìm ñieàu kieän caân baèng. Tìm caùc yeâu caàu cuûa löïc hoaït ñoäng vaø caùc yeáu toá hình hoïc ñeå vaät khaûo saùt ñöôïc caân baèng. 5.2. Phöông phaùp giaûi: Choïn vaät khaûo saùt: Xem xeùt kó moâ hình baøi toaùn (hình veõ), chuùng ta choïn vaät raén naøo (coù theå laø chaát ñieåm) chòu taùc ñoäng cuûa taát caû caùc löïc hoaït ñoäng. 2.2 Ñaët löïc: Löïc ôû ñaây bao goàm caùc löïc hoaït ñoäng vaø phaûn löïc lieân keát. Xem xeùt kó moâ hình vaät khaûo saùt, xaùc ñònh ñaày ñuû caùc lieân keát, so saùnh vôùi caùc moâ hình maãu ñeå thay theá heát caùc lieân keát baèng caùc phaûn löïc töông öùng. Laäp phöông trình caân baèng: a) Phaân tích caùc löïc ñaët vaøo vaät khaûo saùt (keå caû phaûn löïc) theo ba phöông cuûa truïc toaï ñoä. b) Laäp caùc phöông trình caân baèng töø ñieàu kieän caân baèng cuûa heä löïc (keå caû caùc phaûn löïc): F kx = 0 ; 0 ) F ( m M k x ox ; F ky = 0 ; vaø 0 ) F ( m M k y oy ; F kz = 0 ; 0 ) F ( m M k z oz ; c) Vôùi löu yù caùc ngaãu toàn taïi trong heä löïc ñaët vaøo vaät khoâng xuaát hieän trong caùc phöông trình hình chieáu baûo ñaûm vectô chính baèng khoâng.
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