cau 7: trinh bay dinh ly dinh luat lien quan toi' dong nang cua chat diem

a)

dinh ly' dong nang

xet 1 chat diem khoi luong m chiu tac dung cua 1 luc F, chueyn doi tu' vitri 1 toi vi tri 2 cong cua luc F la

A= tich phan F.ds

lai co F= m.a= m. dv/dt

suy ra A= tichphan m.dv.ds/dt

= tichphan m.v.dv

= md((v^2)/2)

A= tichphan d((mv^2)/2)

= m(v2^2)/2 - m(v1^2)/2

Do bien thien dong nang cua 1 chat diem trong 1 quang duong nao do' co gia tri bang cong cua ngoai luc tac dung len chat diem sin ra tu' quang duong do'

Bao toan Dong Nang

neu A=0 suy ra F= 0

thi' W=const

Neu chat diem ko chiu tac dong cua ngoai luc hoac co tong hop cac ngoia luc tac dung len chat diem bang 0 thi dong nang cua chat diem duoc bao toan

DL bao toan Co nang

A= W1t-W2t

A= W2d-W1d

W1t-W2t=W2d-W1d

W1t+W1d=W2t+W2d

W1=W2=const

Khi chat diem chuyen dong trong 1 truong luc the' ma ko chiu tac dong cua 1 luc nao khac thi co nang cua chat diem la 1 dai luong dc bao toan