m1 la khoi luong dai bac
m2 la khôi luong vien dan
v1 la van toc dai bac , v2 la van toc vien dan ;v1=v2=0 (vi luc dau he dung yen)
v1' la van toc dai bac luc sau ; v2' la van toc vien dan luc sau(v2'=400(m/s))
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Ap dung dinh luat bao toan dong luong ta co
m1*v1 + m2*v2 = - m1*v1' + m2*v2' (ban hieu ko vi khi dan ban ra thi khau dai bat giat lui =>nguoc chieu vien dan nen mang dau tru do)
=> 0 = - 4000 * v1' + 10 * 400
=> v1' = 1 (m/s)