^MN = ^ " ' t ^ i - j " " ' ^ ' " (27.3)
1 -) trong dd W^(M) = :r-wi;f
M/^(A^) = ^wi;22
ldn lugt la ddng nang ciia vat tai vj trf ddu M va vj tri eudi V.
Cho bdng nhau hai gia trj eiia A^^^ trong (27.2) va (27.3) ta dugc :
W^ (M) - W^ (N) = W^ (N) - W^ (M) W^iM) + W^{M) = W^{N) + W^{N) Theo djnh nghla ca nang (27.1), ve trai cua edng thiie trdn bieu thj ea nang eiia vat tai M. ve phai bieu thj CO nang ciia vat tai A' :
W{M) = W (N) (27.4) Vi M va N la hai vj trf bat ki ciia vat trong qua
trinh ehuyen ddng, nen tir he thdc (27.4) ed thd' phat bidu djnh luat bao todn ea nang ciia vat chuyen dpng trong trgng trudng :
Khi mdt vqt ehuyen dqng trong trqng trucrng chi chiu tdc dung eua trqng lue thi ecr ndng eua vqt Id mdt dqi luqng bdo todn.
hay
W=W^+W^= hang sd
1 2
:^mv + mgz - hang sd
(27.5)
3, He qua
Trong qua trinh chuyen ddng eiia mdt vat trong trgng trudng :
- Nd'u ddng nang gidm thi the nang tang (ddng nang ehuydn hod thanh the nang) va ngugc lai ; - Tai vj trf nao ddng nang cue dai thi the nang cue tieu va ngugc lai. H I
e
©Hmh 27.2
H I Con lac don tao bdi mdt vat nang nhd gan vao dau mdt sgi day manh khdng co dan, dau kia cua day gan cd dinh tai C (Hinh 27.2). Oua vat len vi tri A rdi tha nhe nhang, vat s§ di xudng de'n O (vi tri thap nha't) rdi di len de'n S, sau dd quay lai va dao dpng cd the tiep dien. Neu khdng co tac dung cua cac luc can, luc ma sat:
a) ChUng minh rang A va B ddi ximg nhau qua CO.
b) Vi tri nao ddng nSng cUc dai ? Cue tieu ?
c) Trong qua trinh nao ddng nang chuyen boa thanh the nang va ngugc lai ?
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B Hmh 27.3
0 B Mdt vat nhd trugt khong 5n td'c dau tu mgt dinh ddc cao
• = 5 m (Hinh 27.3) ; khi xud'ng tdi chan ddc S, van tdc cua vat la v = 6 m/s. Co nang cua vat cd bao toan khdng ? Giai thich.
II - CO NANG CUA VAT CHIU TAC DUNG CUA LUC DAN H b l
Tuang tu nhu trdn cd thd chiing minh rdng : Khi mot vat chi chiu luc dung cua luc ddn hoi gay boi su bien dang eua mot lo xo dan hdi thi trong qud tnnh chuyen dong cua vat, eo ndng ducrc tmh hang tdng dong ndng vd the ndng ddn hoi cua vat la mot dai luong hao todn.
1 7 1 1 . ,
W = ~mv^ + ^k{Aiy = hang sd (27.6) Chil y quan trqng : Djnh luat bao toan ca nang chi nghiem diing khi vat chuydn ddng chi chju tac dung ciia trgng luc va luc dan hdi, ngoai ra nd'u vat con chju thdm tac dung ciia luc can, luc ma sat... thi ca ndng ciia vcit se bien ddi. Cdng ciia cac luc can, luc ma sat... se bdng do bie'n thidn cua ca nang. ffl
^
-Co nang ciia vat chuyein ddng dudi tac dung ciia trong luc bdng tong ddng nang va the nang trong trudng cua vat.
Co nang ciia vat chuyen ddng dudi tac dung ciia luc dan hdi bdng tdng ddng nang va the nang dan hdi cua vat.
Neu khong co tac dung cua luc kbac (nhu luc can. luc ma sat...) thi trong qua trinh chuyein ddng, co nang cua vat la mpt dai luong bao toan.
1
CAU HOI VA BAI TAP
Viet cdng thirc tinh co nang cua vat chuyen dpng trong trpng trudng.
2. Viet cdng thirc tfnh co nang cua vat chiu tac dung cua luc dan hdi.
3. Phat bieu dinh luat bao toan co nang.
4. Neu mpt vi du ve su chuye'n hoa giu'a dpng nang va the nang trong trudng hgp vat chiu tac dung ciia luc dan hdi.
5. Co nang la mpt dai lugng A. ludn ludn duong.
B. ludn luon duong hoac bang khong.
C. cd the duong, am hoac bang khdng.
D. ludn ludn khdc khdng.
6. Khi cd tac dung ciia ca trpng luc va luc dan hoi thi co nang cua vat dugc tfnh nhu the nao ?
144
7. Mpt vat nho dugc nem len tU mpt diem M phia tren mat da't; vat len tdi die'm N thi dUng va roi xudng. Bo qua sire can cua khong khi. Trong qua trinh MN
A. dpng nang tang.
B. the nang giam.
C. CO nang cue dai tai N.
D. CO nang khdng ddi.
Chpn dap an diing.
Tir diem M (co dp cao so vdi mat da't bang 0,8 m) nem len mpt vat vdi van tdc dau 2 m/s.
Biet khdi lugng cua vat bang 0,5 kg, lay g = 10 m/s^. Co nang cua vat bang bao nhieu ? A. 4 J. B. I J .
C. 5 J . D. 8 J .
N A N G L U O N G T H U Y PiEf-.' 6 NUOC T i
6 nudc ta cd nhieu thac cao, nhi^u ddng song bdt nguon td nhung viing nui cao. Nudc d nhirng dp cao dd cd du tru the nang hei sue to Idn. Khi nhung luong nudc dd chay xudng, the nang du tru chuyen boa thanh dpng nSng, lam quay tuabin cua may phat, tao ra dien nang.
Duai day la cong sua't ciia mpt sd nha may thuy dien d nudc ta (hien tai va tuang lai).
Mien
Bdc
Hien tai
h-Tuang lai
Trung
Nam
Hien tai
Tuang lai
Hien tai
Tuang lai
Nha may thuy dien Thac Ba (song Chay) Hoa Binh (song Oa) Son La (song Oa) Lai Chau (song Oa)
Hupi Quang (song Nam /Vlu) VTnh Son (sdng Oa-khan) Song Hinh
Y-a-ly (sdng Se-san) Se-san 3
Oa-nhim
Trj An (song Oong Nai) Thac M a
Ham Thuan Oai Ninh Dong Nai
(1 M w = 10^ yv)
Cdng suat mpt td may (MW) nhan voi sd to may
3 6 . 3 = 1 0 8 M W 240.8 = 1 920 M W
2 400 M W 1 200 M W 600 M W 33.2 = 66 M W 33.2 = 66 M W 180.4 = 720 M W
200 M W 40.4 = 1 6 0 M W 100.4 = 400 M W
1 5 0 M W 400 M W 320 M W 400 M W
10A-VATLI 10 145