Ta tha'y rdng nd'u ehi cd F„ tdc dung thi khdng tao ra ehuyen ddi mong mud'n. Trai lai ehfnh thdnh phdn FJ, cua F da keo eay gd ehuyen ddi theo hudng MN {MN = s).
Ndi each khdc, chi cd thdnh phdn fj. eiia F sinh edng.
Cdng nay ggi la cdng ciia lue F, duge tfnh theo eong thii:c :
A = F^.MN = Fs (24.2) Ggi a la gdc tao bdi luc F va ehuyen ddi MN
(Hinh 24.3), ta cd : F^ = Fcosa
Vi vay, cdng thiie (24.2) ed thd vid't: A - Fscosa.
V
F
1^ ,«
M N
Hmh 24.3
Khi luc T khong doi idc dung len mqt vat va diem dqt cua luc dd chuyen ddi mot doqn s theo hudng hcrp vdi hudng cua lue gdc a tin cdng thuc hien hen luc dd duoe tinh theo edng thdc :
A = Fscosa (24.3)
3. Bien luan
Tuy theo gid tri ciia cosa ta ed cac trudng hgp sau : a) a nhgn, cos« > 0, suy ra i4 > 0 ; khi dd A ggi la edng phdt dcpng.
b) a = 90°, cosa = 0, suy ra .4 = 0 ; khi didm dat ciia lue ehuyen ddi theo phuang vudng gde vdi luc thi luc sinh cdng A = 0.
Chti y :
1. 0 cau hoi dau bai hoc nay. chi co hai truong hop 1 va 3 trong do khai niem "cong" co npi dung dung nhu da dinh nghia.
2. Hai each noi '"liic sinh cong" va
"luc thirc hien cong" la tuong duong.
3. Khi mot vat tac dung lire len mot vat khae va dieim dat ciia lire do chuyen doi ta cung noi vat do sinh cong hoac thuc hien cong.
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N
. . ^ ^ Hmh 24.4
Sk Xac dinh da'u cua cong A trong nhung trUdng hop sau : a) Cong ciia lUc keo cua dpng co 6 to khi d to len ddc ;
b) Cong ctia lUc ma sat cua mat dudng khi 6 to len ddc ;
c) Cdng cua trpng luc cua ve tinh bay vong trdn quanh Trai Oat ; d) Cong cua trgng luc khi may bay cat canh.
\'i dit :
O to CO khdi luong mot tan. chuyen dong deu tren mot duong nam ngang CO he sd ma sat trugt q^ = 0.2. Tfnh cong ciia luc keo ciia dong co va cong ciia lire ma sat khi 6 to chuyen doi dugc 250 m. Cho,? = 10 m/s-.
Giai :
VI 6 to chuyen dgng d6u nen lire keo ciia dgng ca va lire ma sat tren mat duong can bang nhau. Chung co cung dg Idn vit bang :
/.i^mg = 0.2.\ 000.10 = 2 000 N Cong ciia luc keo ciia dgng co :
A^ =f.v = 3.10'^J
Cong ciia luc ma sat (cgng can) : A,=-F.s = ~5.\0^i.
c) a til, cosa < 0, suy ra/\ < 0. Kei qua nay cd y nghia vat If gi ? Ta hay xet mdt d td dang Idn dd'c, mat dd'c nghidng gdc ^ so vdi mat phdng ndrn ngang (Hinh 24.4). Trong chuydn ddi dd trgng luc P ciia d to hgp vdi hudng ehuyen ddi MN gdc a = 90" + ^ > 90°, vay cdng eiia trgng luc nhd hon khdng. _pd giai thfch kd't qua nay ta phan tfeh trgng luc P ra hai thanh phan F^ va P^:
trong dd P^ vudng gdc vdi mat dd'c va P^ song song vdi mat dd'c. Ta nhan thay rdng, thanh phdn P^^
khdng cd tac dung dd'i vdi chuyen ddi MN, cdn thdnh phdn P^ ngucre hudng vdi MN, do dd cd tdc dung cdn trd chuyen dqng.
Kei ludn : Khi-gdc a giu'a hudng ciia luc F va hudng eiia chuyd'n ddi la gdc tii thi luc F cd tac dung can trd chuyd'n ddng va cdng do luc F sinh ra A < 0 dugc ggi la cdng cdn (hay edng am). [ S
4. Don vj cong
Don vi edng la jun (kf hidu la J). Trong (24.1) ne'u F = 1 N va 5' = 1 m thi :
/ \ = 1 N.l m = 1 N.m= 1 J
.fun Id cong do luc cd do km IN thuc hien khi diem deii ciia life chuyen ddi 1 m theo hudng ciia life.
5. Chu V
Cdc edng thue tfnh cdng (24.1) va (24.3) chi dung khi diem dat ciia luc chuydn ddi thdng va luc khdng ddi trong qua trinh chuyd'n ddi.
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II - C O N G SUAT
1. Khai niem cong suat
Trong sdn xuat va ddi sd'ng, ngudi ta thudng sir dung cae loai may mdc, ddng co, tdng qudt hon la cae thiet bi sinh cdng (edng duong). Khi dd ngodi do ldn ciia cdng do thid't bi sinh ra, ngudi ta edn quan tam de'n khodng thdi gian thuc hien cdng dd.
Ciing sdn ra mdt cdng, thidt bi nao thue hidn trong thdi gian ngdn hon .se ldm vide manh hon. Ndi each khae ngudi ta danh gia mire do manh eiia mdt thie't hi sinh edng bdng do ldn eiia cdng do thid't bj dd thue hidn trong cimg mdt khoang thdi gian chgn trudc - thudng chgn la don vi thdi gian. Dai lugng do dugc ggi la cdng sudt hay tdc do sinh cdng.
Cong suat Id dqi luong do bang edng sinh ra trong mot dim vi thcri gian.
Nd'u trong khoang thdi gian t cdng sinh ra bdng A{A > 0) thi cdng suat (kf hidu ./) duge tfnh theo cong thirc :
t (24.4)
Cung cd the ndi rdng, edng suat eua mdt luc do tde do sinh cdng ciia luc dd.
2. Oon vj cong suat
Don vi cdng suat la jun/giay, duge dat tdn la oat, kf hieu W.
1 W = 1 s
Oat la edng sudt eiia mdt thiet bi thue hidn edng bdng 1 J trong thdi gian 1 s. I S
Ngudi ta cdn sir dung mdt don vj thue hanh eiia edng la oat gid (W.h) :
1 W.h = 3 600 J 1 kW.h = 3 600 kJ
S B So sanh cdng sua't cua cac may sau ;
a) Can cau M.^ nang dugc 800 kg len cao 5 m trong 30 s ;
b) Can cau /Wj ^lang dugc 1 000 kg len cao 6 m trong 1 phiit.
Chu y: TrUdc day ngudi ta cdn dimg don vi ma luc de do cdng sua't.
0 nudc Phap ;
1 ma luc = 1 CV = 736 W.
6 nudc Anh :
1 ma luc = 1 HP = 746 W.
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Bang 24,1
Vai vi du ve cong sua't trung binh.
Ten lira Satan (Saturn) Tau bien
Dau tau hoa Oto Xe may Ngudi lam viec Den dien May thu thanh May tinh bo tiii
V 7,1010W 5.10^ W 3.106 W 4.10''W
1,5.10''W 100 W 100 W 10W
10-3 W
3. Khdi nidm cdng sua't cung dugc md rdng cho cae nguon phdt ndng luang khdng phdi difdi dqng sinh cdng cahqc. Vidu : Id nung, nha may didn, dai phat sdng..., xem Bang 24.1.
Ngudi ta eung djnh nghTa edng sua't tidu thu eiia mdt thiei hi deu thu ndng luqng la dai lucmg do bang nang lugng tidu thu ciia thid't bj dd trong mdt don vj thdi gian.
Neu luc khong ddi F co diem dat chuyen ddi mdt doan s theo hudng hop voi hudng cua luc goc a thi cong cua luc F duoc tinh theo cdng thuc :
A = Fscosa
Cong suat do bang cong sinh ra trong mdt don vi thdi gian.
Ik.
^ ^ A
CAU HOI VA BAI TAP
f l 4. Cdng co the bie'u thi bang tfch cue '* ""• A. nang lugng va khoang thdi gian.
1. Phat bieu dinh nghTa cong va don vi cdng. B. luc, quang ducmg di dugc va khoang then gian.
Neu y nghla cua cong am. C. luc va quang dudng di dugc.
D. luc va van tdc.
2. Phat bie'u dinh nghla cdng sua't va don vi cdng Chon dap an diing.
sua't. Neu y nghla vat li ciia cd; ,g sua't ? •- . . , - ,,_. j . ' - , . . . ,- .. -.
5. Mpt luc F khong doi lien tuc keo mpt vat chuye'n dpng vdi van tdc i7 theo hudng cua
F. Cdng suat cua luc F la 3. Don vi nao sau day khdng phai la don vj . p , • p
cong suat ?
C. Ft. D. Fu2, A. J.s. B. W.
C. N.m/s. D. HP. Chon dap an diing.
132
6. Mpt nguoi keo mdt hdm gd khd'i lugng 80 kg trugt tren san nha bang mpt day co phuang hgp goc 30° so vdi phuong ndm ngang. Luc tac dung len day bang 150 N. Tinh cdng ciia luc do khi hdm truot di duac 20 m.
Mdt dpng co dien cung cap cong sua't 15 kW cho mot can cdu nang 1 000 kg len cao 30 m. Lay g = 10 m/s^. Tinh thdi gian td'i thieu de thuc hien cdng viec d6 ?
Em c6 bi'
HOP SO O T O . XE M A Y
Trong 6 to, xe may cong suai ciia luc phat dpng F cho bai : -f = — A
Gia sif diem dat ciia f chuyen dai mot doan As theo hucVng ciia F , cong .'AA ciia F la : A/\ = FAs d o d o :
^ = F —
•^ Ar As .
Vai Af nho, — la van toe tuc thai v ciia 6 to, xe may tai thai diem dang xet. Vay : - / = Fv {*}
Thuang cong suai ciia dpng c o 6 to, xe may la mot dai luang duac duy tri khong d o i . D o do, theo (*) neu F t a n g thi v giam va ngugc lai.
N h u vay, khi 6 to, xe may chay qua nhupg doan duang kho di (ien doc, ma sat lan) thi cudng dp luc F p h a i tang len, do do van toe v phai giam. Nguac lai, 6 nhCrng doan d u a n g de di (xuong doc, ma sat nho) cuang dp lire F giam va van toe v se tang. Viec dii;u c h i n h v tang hay giam dope thuc hien bang mpt thiei bi gpi la hop sdisu dung cac banh xe truyen dpng co ban kinh to, nho khae nhau).
£) O
Hop so xe n-iay
Hop so 6 to •
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