iJ TlNH TUONG D 6 I CUA
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11 - CONG THUC CONG VAN TOC
1. He quy chieu dung yen va he quy ehieu chuyen dpng
Mdt chide thuydn dang chay trdn mdt ddng sdng.
Ta se xet chuyen ddng cua thuydn trong hai he quy chidu :
- He quy chid'u (xOy) gan vdi bd coi nhu he quy chie'u dijmg ydn (Hinh 6.2a).
- He quy chid'u (x'O'y') gan vdi mdt vat trdi theo ddng nudc la he quy chid'u chuyen ddng (Hinh 6.2b).
"nb
Hmli
Vi du : ne'u v , = 2 km / h,
nb
D,^ = 30km/hthi i;,^ = 2 8 k m / h .
2. Cong thtrc cong van toe
a) Trudng hap cdc vein tdc cUng phuang ciing chieu Thuydn chay xudi ddng nudc.
Ggi i^tb la van td'c cua thuydn dd'i vdi bd, tiic la ddi vdi he quy chie'u diing yen. Van td'c nay ggi la vein tdc tuyet ddi.
Ggi i^tn la van tdc cua thuyen dd'i vdi nudc, hie la dd'i vdi he quy chidu chuydn ddng. Van td'c nay ggi la vein tde tuang ddi.
Ggi i^nb Is van tdc cua nudc dd'i vdi bd. Dd la van td'c cua he quy chie'u chuyen ddng so vdi he quy chieu diing yen. Van tdc nay ggi la van tdc keo theo.
Dd dang tha'y rang : ^tb ^ ^m + i^nb(Hinh 6.3).
He thiic nay cd the vid't dudi dang :
(6.1)
I'U ^ ^1,2 + ^2,3
Trong do : sd 1 iing vdi vat chuyen ddng ; umg vdi hd quy chid'u chuydn ddng ; sd 3 iing vdi he QUV chid'u diing ven.
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b) Trudng hop van tdc tuang ddi cdng phuang, nguac chieu vdi vein tdc keo theo
Thuydn ehay ngugc ddng nudc. Vecta van td'c tuomg dd'i ?tn se ciing phuang, ngugc chieu vdi vecta van td'c keo theo v^^ (Hinh 6.4).
Vd do ldn, rd rang la van td'c cua thuyen dd'i vdi nudc phai trii di van td'c chay cua ddng nudc mdi thanh van td'c ciia thuydn dd'i vdi bd :
KbI = If^tnl - l^nbl
Tuy nhidn, dudi dang vecto, ta van phai vidt :
^tb = ^tn + V nb
(?,b la tdng ciia hai vecta ciing phuang, ngugc chidu) S 3
Nhu vay cdng thufc (6.1) cd tfnh tdng quat. Dd la cong thitc cdng van tdc. Vecta vein tdc tuyet ddi bdng tdng vecta ciia vein tdc tuang ddi vd van tdc keo theo.
Vtb Vr,b
Hmh 6.4
t ^ Mpt con thuyen chay ngugc dong nude di duge 20 km trong 1 gid ; nudc chay vdi van tde 2 km/h. Tfnh van tde eua thuyen ddi vdi nude.
S-f"
Quy dao va van tdc cua ciing mdt vat chuyein ddng ddi vdi cac he quy chieu khae nhau thi khae nhau.
Cdng thdc cong van tdc : Vecto van tdc tuyet ddi bSng tdng vecto ciia van tdc tirong ddi va van tdc keo theo : F = r, + T\ ,.
Van tdc tuyet ddi la van tdc ciia vat ddi vdi he quy chieu dung yen ; van tdc tirong ddi la van tdc ciia vat ddi vdi he quy chieu chuyetn ddng ; van tdc keo theo la van tdc ciia he quy chieu chuydn ddng ddi vdi he quy chieu dung yen.
CAU HOI VA BAI TAP
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1. Neu mpt vf du ve tfnh tucmg ddi ciia quy dao cua chuyen dpng.
2. Neu mpt vi du ve tfnh tuong ddi ciia van tdc cua chuyen dpng.
Trinh bay cdng thUc edng van tdc trong trudng hgp cac chuyen dpng eung phuang, cung ehieu (ciing phuong va ngugc chieu).
Chpn eau khang dinh diing.
Dirng d Trai Oat, ta se tha'y
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A. Mat Trdi dUng yen, Trai Oat quay quanh Mat Trdi, Mat Trang quay quanh Trai Da't.
B. Mat Trdi va Trai Da't dirng yen, Mat Trang quay quanh Trai Da't.
C. Mat Trdi dirng yen, Trai Oat va Mat Trang quay quanh Mat Trdi.
D. Trai Da't dirng yen. Mat Trdi va Mat Trang quay quanh Trai Oat.
Mpt chie'c thuyen buom chay ngugc ddng song, sau 1 gid di dugc 10 km. Mpt khue gd trdi theo ddng sdng, sau 1 phiit trdi dugc
— - m. Van toe ciia thuyen buom so vdi nudc bang bao nhieu ?
A. 8 km/h. B. 10 km/h.
C. 12 km/h. D. Mpt dap sd khae.
6. Mdt hanh khach ngoi trong toa tau H, nhin qua cira sd tha'y toa tau N ben canh va gaeh lat san ga deu chuyen dpng nhu nhau. Hoi toa tau nao chay ?
A. Tau H dirng yen, tau N chay.
B. Tau H chay, tau N dirng yen.
C. Ca hai tau deu chay.
D. Cac cau A, B, C deu khdng dung.
7. Mpt 6 to A chay deu tren mpt dudng thing vdi van td'c 40 km/h. Mpt d td 6 dudi theo oto A vdi van tdc 60 km/h. Xac dinh van tdc cua 6 td 6 ddi vdi 6 td >4 va ciia oto A ddi vdi d td 6.
8. A ngoi tren mpt toa tau chuye'n dpng vdi van tdc 15 km/h dang rdi ga. 6 ngoi tren mpt toa tau khae chuyen dpng vdi van toe 10 km/h dang di ngugc chieu vao ga. Hai dudng tau song song vdi nhau. Tfnh van td'c cua 6 ddi vdi A.
Em c6 biet ?
\'\N TOC ANH SANG
Mpt 6 to dang chay vai van toe v thi bat den pha (Hinh 6.5). Doi vai ngudi lai xe, anh sang truyen di vai van toe c{c= 3.10" m/s). Odi voi ngudi dung ben le duang thi co le anh sang se CO van toe c + u.
Khong dau. Can cir vao cae thi nghiem rai chinh xac ma nhieu nha bae hoc loi lac da tien hanh vao cuoi the ki XIX de nghien ciru su truyen anh sang trong eae moi truang, Anh-xtanh (Einstein) da di den kei luan la, van toe anh sang ddi voi mpi he quy chieu khae nhau la nhu nhau va deu bang c.
Cong thuc cpng van toe ma ta hoc d day khong dung cho truang hpp cac vat chuyen dpng vai van tdc rai lan (so sanh duac vai van toe anh sang). Cae em se biei ro dieu nay
trong Thuyet tuang doi cua Anh-xtanh (1905). Hinh 6.5
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