Cau 2

a) Dinh luat bao toan dong luong

xet 1 chat diem chuyen dong

[d(m1v1 + m2v2+...+mnvn)]/dt = F

F la tong hop cac luc tac dung

He co lap F= 0

suy ra m1v1+m2v2+..+mnvn= const

Neu he khong co lap

F#0 nhung hinh chieu cua F len 1 phuong nao do' = 0

thi'm1V1x+m2v2x+....= const

b) ptcuyen dong cua vien dan ban' xuong la

x= V.sina.t

y=Vcosa.t+gt^2/2

thoi gian cham dat'

y=h=gt^2/2+v.scosa.t

van toc luc cham dat

v=can(vx^2+vy^2)

vx= V.sina

vy=Vcosa+gt