The conservation of momentum of a system is a fundamental principle in classical mechanics. The only condition for this principle being valid is that the system should be an isolated one, i.e. It should not be acted upon by any external force.
Max preassure = force / min area = 3N / 0.1 x 0.05 = 600N/m(squared) Copy off of the picture below itll help better, its what someone sent me when i asked this question