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From Physics (Mechanics), there is this impulse-momentum change equation:
= F*t, ( http://www.physicsclassroom.com/class/momentum/Lesson-1/Momentum-and-Impulse-Connection )
where m is the mass of a body, F is a force acting to the body, t is time and = is the change of velocity.
We consider everything is happen along a straight line, and gravitation does not participate.
So, the increase of momentum is F*t = 10000 N * 60 seconds = 600000 N*s = 600000 (kg*m)*s/s^2 = 600000 kg*m/s.
Increase of velocity is = = = 500 m/s.
New velocity after the engine was firing during 60 seconds is 2000 + 500 = 2500 m/s.
Solved.