Conservation of Momentum - Physics 101 / AP Physics 1 Review with Dianna Cowern

@14:56: "the rocket goes up with the same change in momentum".  Let's say the rocket is massive and the gas being ejected are not of sufficient velocity to apply a force necessary to overcome the force of gravity. So all you've done is to reduce the contact force of the rocket with the ground, but the rocket would not launch and hence it's vertical velocity would still be zero and its momentum would still be zero. The issue as I see it is that you don't have a closed and isolated system and thus the conservation of momentum would not apply. But perhaps you can explain to me why I'm wrong.