Phöông trình caân baèng laäp caùc phöông trình caân baèng töø F k Caùc ñieàu

Phöông trình caân baèng laäp caùc phöông

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Phöông trình caân baèng: laäp caùc phöông trình caân baèng töø 0 F k . Caùc ñieàu kieän (ma saùt): - Khoâng tröôït F m fN. - Khoâng laên M kN. Bieän luaän caùc khaû naêng: - Ñieàu kieän vaät khoâng laên vaø khoâng tröôït. - Ñieàu kieän vaät laên khoâng tröôït. - Ñieàu kieän vaät tröôït khoâng laên. 7.2. Ví duï : Con laên ñoàng chaát baùn kính R, troïng löôïng P chòu taùc duïng cuûa löïc Q nhö hình veõ. Bieát heä soá ma saùt tröôït vaø laên giöõa con laên vaø maët ñöôøng töông öùng laø f, k. Tìm giaù trò cuûa Q ñeå con laên caân baèng. Giaûi: Trong caùc baøi toaùn loaïi naøy, ta khoâng aùp duïng ñöôïc caùc phöông trình caân baèng tôùi haïn, bôûi vì caùc ñieàu kieän laên vaø tröôït laø ñoäc laäp, khoâng baét buoäc cuøng ñaït tôùi ñieàu kieän tôùi haïn. - Vaät khaûo saùt: con laên. - Ñaët löïc: 0 M , F , N , Q , P F m k . - Phöông trình caân baèng: 0 Qh M m 0 P N F 0 F Q F I ky m kx M l Xu höôùng tröôït Hình 2.11 N ms F k F n F S Xu höôùng laên P Hình 2.12 m F M Q Xu höôùng laên N I R O h y x
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Baøi giaûng moân Cô lyù thuyeát Hoïc kyø 2, naêm hoïc 2012-2013 23 - Giaûi heä phöông trình: Giaûi 3 phöông trình ñaàu chuùng ta nhaän ñöôïc: N = P ; F m = Q ; M = Qh. Khoâng tröôït: Q fP. Khoâng laên: Qh kP Q Pk/h. Ñeå con laên caân baèng (khoâng laên, khoâng tröôït): ) h k , f min( P Q . Chuù yù: giaû söû h k < f, chuùng ta coù caùc tröôøng hôïp: f P Q h k : vaät laên khoâng tröôït. P Q f h k : vaät vöøa tröôït vöøa laên.
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