Mot dong tu xuat phap tu A tren duong thang huong ve B voi van toc ban dau \(v_0\)=1 m/s bt rang cu sau 4s chuyen dong van toc lai tang gap 3 lan va cu chuyen dong dc 4s thi dong tu ngung chuyen dong trong 2s . Trong khi chuyen dong thi dong tu chi chuyen dong thang . Sau bao lau chuyen dong tu den B bt AB dai 6 km?