Một hệ gồm 2 vật có khối lượng m 1=500g, m2=800g có vận tốc v1=6m/s và v2=4m/s. Tính độ lớn động lượng của hệ trong các trường hợp A) v1 cùng hướng với v3 B) v1 ngược hướng với v2 C) v1 vuông góc với v2 D) v1 hợp với v2 góc 60 độ

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