Mmm tricky. Since the velocity is constant, I'm going to assume there is no acceleration in any direction. This means there is no net force in the I or J forection!
Since there are 2 forces, both must be equal and opposite in direction to perfectly cancel each other out.
You can use the impulse momentum theorem and just subtract the two momenta. P1 - P2 = (16-1.2)(11.5e4)=1702000Ns If you first worked out the force and integrated it over time the result is the same
