During a test flight in which a 9000-kg helicopter starts from rest at t = 0, the accelera...

During a test flight in which a 9000-kg helicopter starts from rest at t = 0, the acceleration of its center of mass from t = 0 to t = 10 s is

a = 0.6ti + (1.8 – 0.36t).j (m/s2).

What is the magnitude of the total external force on the helicopter (including its weight) at t = 6 s?