Assuming that there is no friction anywhere and both box 1 and ramp 2 start at rest, I was wondering why this is an isolated system in terms of momentum calculation. My professor approached calculating the velocities of the box and the ramp assuming that the momentum is conserved before and after the release of box 1. However, I don't understand how the system can be treated as if there is no net force acting on the system. If we draw the two axes horizontal and vertical to the surface of ramp 2, then the vertical component of gravity is canceled out by the normal force but the horizontal component of gravity remains unhindered and this is precisely why box 1 will be moving at all. I'm having a hard time understanding how the conservation of momentum approach can be applied here.