Cau 4

Dinh nghia va y nghia cua momen dong luong. trinh bay ca dinh ly va dinh luat ve momen dong luong

a) Dinh nghia:

xet 1 chat diem M chuen dong tren 1 quy dao (C) duoi tac dung cua luc F tai 1 thoi diem nao , chat diem o m xac dinh boi OM = r va no co dong luong K= m.v

khi do dai luong L= r..k goi la momen dong luong cua 1 chat diem doi voi o

vay : momen dong luong cua 1 chat diem la 1 dai luong vecto xd bang tich co' huong giua vecto ban kinh quy dao va vecto dongluong cua chat diem do'

voi chat diem chuyen dong tron : L= I.w

voi he chat diem : L=tong Ri.Ki

b) dinh ly ve momen

dK/dt=F

r..dk/dt=r..F

d(r..k)/dt=r..F

dL/dt=Mo(F)

vay: dao ham theo thoi gian cuamomen dong luong doi voi cua 1 chat dioemj chuyen dong bang tong momen doi voi O cua cac luc tac dung len chat diem voi chat diem

Voi chuyen dong tron

d(I.w)/dt= Mo(Ft)

Ft la luc tt

Tu' dL/dt = Mo(F)

dL=Mo(F)dt

tich phan dL tu l1 toi l2 = tich phan Mo(F)dt tu1 t1 toi' t2

deltaL=tich phan Mo(F)dt

neu Mo(F) khong doi suy ra deltaL = Mo(F).deltat

vay:

do bien thien momen dong luong rtrong 1 khoang thoi gian bang xung luong cua momen luc tac dung len chat diem trong khoang thoi gian do'

c)

Dinh luat momen dongluong

tu' dL/dt = M

suy ra neu M= 0 thi' dL/dt = 0 suyra L= const

Vay

Doi voi 1 he co chat diem co lap hoac chiu tac dung cua cac ngoia luc nhung tong momen cua cac ngoai luc ay' doi voi goc toa do bang o , thi tong mom dong luong cua he la 1 dai luong dc bao toan

Neu he chat diem chuyen dong xung quanh 1 truc co dinh

d(I1w1+I2w+....) = M=0

suy ra (....)= const

suy ra vay ran quay xunh quay 1 truc co dinh m=0

suy ra Iw= const