BA LUC KHONG SONG SONG
Trong dai song va ki thuat chung ta thuang gap nhung vat ran. Do la nhirng vat co kich thuac dang ke va hau nhu khong bi bien dang duai tac dung ciia ngoai luc. Viec xet su can bdng ciia vat ran mang lai nhung kei qua co y nghia thi/c tien to lan.
I - CAN BANG CUA MOT VAT CHIU TAC DUNG CUA HAI LUC
Htnh 17.1
S3 Cd nhan xet gi ve gia cua hai luc F., va F2 khi vat dUng yen ?
1. Thi nghiem
Thi nghiem dugc bd trf nhu d Hinh 17.1. Vat la mgt chie'c vdng hay mdt midng bia cirng va nhe. Hai rdng roc cd true quay ndm ngang va song song vdi nhau. -'Ml
Thi nghiem cho tha'y, vat dung yen neu hai trgng lugng /*[ va P , bdng nhau va ne'u hai day budc vao vat ndm tren ciing mdt dudng thdng. Hai day nay cu the hod gid ciia hai veeto luc F^ va Fj.
2. Oieu kien can bang
\luon cho iiiol vot chiu tdc dint'.; ctia nai life ir uaii'j thai can hang thi hai hu J<> ohcti •'•uu<; gid,
line do Urn ret nvifoc cl-tci.
F = -To (17.
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3. Cach xac dinh trong tam cua mot vat phang, mong bang phuong phap thuc nghiem
Nhu da bie't, trgng tam la diim dat eiia trgng luc ciia vat. Dua vao dieu kien can bdng tren day. ta ed the dua ra each xdc djnh trgng tam eua vat phdng, mdng nhu sau :
Budc day vao mdt Id nhd A a mep eiia vat rdi treo nd len (Hinh 17.2). Vat ddng yen dudi tde dung cua hai luc can bdng : trgng luc ciia vat dat tai trgng tam va lue eang ciia day dat tai diem A. Do dd, trgng tam ciia vat phai ndm tren dudng keo ddi ciia day treo, tue la dudng AB tren vat. Sau dd, bude day vao mdt di6m khae C d mep vat rdi treo vat len. Khi ay, trgng tam phdi ndm tren dudng CD. Nhu vay, trgng tam G la giao diem eiia hai dudng thdng AB va CD.
Hinh 17.3
Thi nghiem edn cho tha'y, trgng tam G eiia cdc rag Em hay lam nhu Hinh 17.3 vat phdng, mdng va ed dang hinh hgc dd'i xiing ndm va cho bie't trgng tam cua thudc d tam dd'i xumg eiia vat (Hinh 17.4). L S det d dau.
. G G
. <iiiii I . ' .i
7 A - V A T Ll 10 97
Hmh 17.5
Sl C6 nhan xet gi ve gia cua ba luc?
li - CAN BANG CUA M O T VAT CHjU TAC DUNG CUA BA LUC KHONG SONG SONG
1. Thi nghiem
Dung hai luc ke (gdn vao bang sdt) treo mdt vat phdng mdng, ed trgng lugng P va trgng tam G da bie't. Hai lue ke cho bie't do ldn eua hai lue cang, cdn hai day eho bie't gia eiia hai lue dd (Hinh 17.5).
Diing mdt day dgi di qua trgng tam di cu the' hod gid ciia trgng luc. S 3
Thf nghiem cho thay, gid ciia ba luc cung ndm trong mdt mat phdng.
Diing mdt cdi bang de cu the hod mat phdng va ve tren bang ba dudng thdng bieu diin gid ciia ba luc. Ta nhan thay, ba gid dong quy tqi nwt diem.
Ve tren bdng ba vecta Fj, Fj va P(Hinh 17.6a) theo mdt ti xfch quy udc rdi trU0 cdc vecta luc tren gid ciia chiing de'n diem ddng quy O, ta dugc he ba lue can bdng gid'ng nhu d chat diem (Hinh 17.6b).
2. Quy tac tong hop hai luc co gia dong quy Mudn tdng hop hai luc cd gid ddng quy tdc dung len mcJt veit rdn, trudc hei ta phdi truqt hai vecto luc dd tren gid eua chimg den diem ddng quy, rdi dp dung quy tde hinh binh hdnh de tim hop luc.
3. Dieu kien can bang cua mot vat chiu tac dung cua ba luc khong song song
Mudn cho mdt vat chiu tdc dung eiia ba \ucF^,F,F^
khdng song song d trang thdi can bdng thi :
- Ba life dd phdi cd gid dong phdng vd dong quy ; - Hap life ciia hai life phdi cdn bdng vdi life tint ba.
Fx+F2 - F , (17.2)
98 / B - V A T Li 10
b)
Hinh 17.7 Hinh 17.8
Vi dit :
Mot quik cau d6ng cha't co trpng luong 40 N duoc treo vao tuong nho mpt spi day (Hinh 17.7). Day lam voi tuong mpt goc a - 30". Bo qua ma sat is ch6 tiep xuc ciia qua cau vcji tucmg.
Hay xac dinh lire cang ciia day va luc ciia tudng tac dung len qua cau.
Gidi :
Qua cau chiu tac dung ciia ba lire : Trpng lire P, lire eang T ciia day va iuc N ciia tuong. Do bo qua ma sat nen lire N vuong goe vcri tuong. Vi qua cau dung yen nen ba lire nay phai d6ng phang va dong quy tai tam O ciia qua eau (Hinh 17.8a).
Ta trupt ba lire tren gia eiia chiing den diem dong quy, roi thuc hien phep t6ng hpp lire nhu da lam doi vol chat diem. Tii cac tam giac lire (Hinh 17.8b).
ta CO :
N = P t a n a = 40tan30" ~ 23 N.
T = 2N~ 46 N.
Dieu kien can b^ng ciia mdt vat chiu tac dung ciia hai luc la hai luc dd phai ciing gia, ciing dp Idn va nguoc chieu.
Oieu kien can b^ng ciia mpt vat chiu tac dung ciia ba luc khdng spng spng:
~ Ba luc dd phai cd gia ddng ph^ng va ddng quy.
- Hpp luc ciia hai luc phai can bang vdi luc thu ba.
Quy tac tdng hpp hai luc cd gia ddng quy :
Mupn tdng hpp hai luc cd gia ddng quy, trudc het ta phai trupt hai vectp luc dd tren gia ciia chiing den diem ddng quy, rdi ap dung quy tSc hinh binh hanh de tim hpp luc.
CAU HOI VA BAI TAP
Wm^.
1. Phat bieu dieu kien can bang ciia mpt vat ran chiu tac dtjng ciia hai luc.
2. Trong tam cita mot vat la gi ? Tnnh bay phuong phap xac dinh trpng tam ciia vat phdng, mong bang thuc nghiem.
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3. Cho bie't trpng tam cua mpt sd vat dong chat va co dang hinh hoc doi xiJng.
4. Phat bieu quy tac tdng hgp hai luc dong quy.
5. Dieu kien can bang ciia mpt vat chiu tac dtjng ciia ba luc khdng song song la gi ?
Mpt vat CO khdi lugng m = 2 kg dugc giu yen tren mpt mat phang nghieng bdi mpt sgi day song song vdi dudng ddc chinh (Hinh 17.9).
Bie't goc nghieng a - 30° g = 9,8 m/s^ va ma sat la khdng dang ke. Hay xac djnh :
a) luc cang cua day ;
b) phan luc cua mat phdng nghieng len vat.
m = 2kg
Hmh 17.9
7. Hai mat phang dd tao vdi mat phang nam ngang cac goc a = 45°. Tren hai mat phang do ngudi ta dat mpt qua cau dong chat co khoi lugng 2 kg (Hinh 17.10). Bo qua ma sat
va lay g = 10 m/s^. Hoi ap luc cua qua cau len mdi mat phang dd bang bao nhieu ? A. 20 N ; B. 28 N ; C. 1 4 N ; D. 1 4 N .
Hinh 17.10
8. Mpt qua cau dong chat co khoi lugng 3 kg dugc treo vao tudng nhd mpt sdi day. Day lam vdi tudng mpt goc a = 20° (Hinh 17.11). Bo qua ma sat d ch6 tiep xiic cua qua cau vdi tudng, lay g = 9,8 m/s^.
Luc cang Tciia day la bao nhieu ? A. 88 N ; B. 10 N ;
C. 28 N ; D. 32 N. Hinh 17.11
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