Hoat dgng 1
Kiem tra, chuan bi dieu kien ; xuat phat. Dat van de
HS suy nghT ca nhan, sau đ thao ;
luan chung toan Idp. : - Trudng hpp may mai quay nhanh ;
thi dTa may se cd nang lupng Idn : hon vi cd kha nang sinh cdng lam | mai mdn cae vat va gay ra su toa •
nhiet... • Ca nhan nhan thirc dupe van d l |
ciia bai hpc. •
Hoat dgng 2
Xay dung bieu thiic tinh dgng | nang cua vat rdn quay quanh ; mgt true cd dinh
HS suy nghT ca nhan. i
T r g giiip cua giao v i e n
GV neu cau hdi kiem tra kién thiic cu: - So sanh nang lupng ciia dTa may mai trong hai trudng hpp : may mai quay nhanh va may mai quay cham.
Dgt van de : Nang lupng cua dTa may
mai khi quay gpi la đng nang cua vat rin (dTa mai) quay quanh true cd dinh. Bilu thiic tinh đng nang cua vat rin quay quanh true cd dinh dupc xac dinh nhu the nao ? Bai hpc ngay hdm nay giiip chiing ta xay dung bilu thiic đ.
GV yeu cau HS tim bieu thu:c tinh đng nang ciia vat rin quay quanh mdt true CO dinh. A ' ^ y ^ ] / ^ V 1 m, \ 1
Hoat dgng cua hgc sinh
Cac chai dilm chuyin đng quay vdi ciing tdc đ gdc, đng nang cua mdt chat dilm dupe xac dinh :
-w,v,. =-m,{cor^)
Ddng nang cua vat rin bing tong đng nang cua cac chat dilm cau thanh vat:
^^ V / \2 1 , 2
Hoat dgng 3
Xay dung bieu thurc ciia djnh If bien thien dgng nang ciia vat ran quay quanh mgt true cd djnh
HS suy nghT ca nhan.
HS lam viec ca nhan vdi phieu hpc tap.
Cdng cua ngoai luc la :
A = Fs^ FRcp Mat khac M = FR = Ịy
=> /4 = lycp.
Ma ta lai co; 0)2-0)^ - 2Ycp ^A = -lcol—lcỏ = AW
2 ^ 2 '
Trg giiip cua giao vien
GV neu cac cau hdi gpi y:
- Coi vat rin gdm nhilu chat dilm, mdi chat dilm cd khd'i lupng m; va each true quay mdt khoang /•;. Van tdc gdc ciia cac chat dilm nay cd dac dilm gi ? - Ddng nang ciia mdi chat dilm dupe xac dinh nhu the nao ?
- Ddng nang cua vat rin dupe xac djnh nhu the nao ?
GV neu cau hdi vl van dl can nghien ciru: - Khi vat rin quay quanh mdt true cd
dinh chiu tac dung cua ngoai luc F
khdng doi va ludn tiep tuyen vdi quy dao dilm dat thi đng nang cua vat rin thay doị Dp bién thien đng nang ciia vat rin đ cd quan he nhu the nao vdi
cdng cua ngoai luc F ? Bilu thiic toan
hpc nao bilu thi mdi quan he đ ? GV neu cac cau hdi gpi y:
- Yeu eau HS lam viec vdi phieu hpc tap dl tra Idi cau 1.
- Cdng cua luc F dupe xac dinh nhu the
nao ?
- Cdng ciia lire F cd quan he nhu the
nao vdi mdmen ciia luc F ?
- Viei bilu thiíc lien he giiia tdc dp gdc, gia tdc gdc va dp bién thien toa dp gdc.
Hoat d g n g cua hgc s i n h HS tilp thu, ghi nhd.
Hoat dgng 4
Ap dung cdng thurc tinh dgng nang ciia vat ran quay quanh mot true cd dinh de lam bai tap HS lam viec ca nhan vdi phieu hpc tap.
Ddng nang liic diu cua vat la : f^d(dau)=^/l^'=202,5J. Vi mdmen lire tac dung vao van đng vien bing 0 nen theo dinh luat bao toan mdmen đng lupng, ta cd :
/|<yi - I2CO2 => 6^2 = 3^1
Ddng nang luc cud'i la:
f ^ d , c u 6 i ) = ^ / 2 ^ 2 ' = ^ y ( 3 ^ l ' f = 3.^d(dau)=607,5J. Hoat dgng 5
Cling cd bai hgc va dinh hudng nhiem vu hgc tap tiep theo
T r g g i u p cua giao v i e n GV thdng bao:
- Bilu thiic A = -Io)j--lo}? =AW la
2 ^ 2
bilu thiie ciia dinh li bién thien đng nang.
- Dinh li bién thien dgng ndng : Dd bién
thien đng nang cua mdt vat bing tdng cdng cua cae ngoai lire tac dung vao vat.
GV yeu cau HS lam viec vdi phieu hpc tap dl lam bai tap 2.
GV neu cac cau hdi gpi y:
- Xac dinh đng nang bing cdng thirc nao ? Ddng nang ban diu dupc xac dinh nhu the nao ?
- Mud'n xac dinh dupc đng nang liic cud'i can phai xac dinh dupc dai lupng nao ?
- Theo dinh luat bao toan mdmen đng lupng, ta cd thi tinh dupc tdc dp gdc liic cud'i nhu the nao ?
GV yeu cau hpc tim su tuang tu giira bilu thiic đng nang ciia vat rin quay
Hoat đng cua hgc si^h
HS lam viec ca nhan, sau đ thao luan chung toan Idp.
Trg giiip cua giao vien
quagh mgt true cd dinh vdi bilu thúc đng nang cua vat chuyin đng tinh tig'n va lam cae bai tap cdn lai trong phieU hpc tap.
- Vl nha lam cac bai tap trong SGK. - On cac kién thiic, eac cdng thiic va ,phuang trinh đng lire hpc cua chuyin
đng quaỵ
- On lai phuang phap đng lire hpc da hoc d Idp 10 THPT.