A body with a mass of M is on a friction-free horizontal floor. The shape of the body is a half circle with a radius of R. A mass of m is put at the left edge of M and is released from rest. the mass m rolls on the half circle with no friction. Is there conservation of momentum during the movement in respect to the system? Well, in the solution they say that for the horizontal x axis there is, since no external forces are applied on the system in the x axis. But they say that for the vertical y axis there is no conservation of momentum but I don't understand their explanation: They draw the forces on M and m (I uploaded this draw) for a moment where m is at something like half the distance between its start point and the bottom of the circle point. They say that because N is an internal force in the system, it doesn't affect the external impulse. And then they say that Nfloor cancells Mg and because there is no external force to cancel mg, you get that for the y axis the external impulse isn't 0 and so there is no conservation of momentum for the y axis. I don't understand this analysis: How can they on one hand - decide to look at the system where N is an internal force, but on the other hand to determine that Nfloor cancells out Mg?? I mean if anything, Nfloor cancells out (M+m)g if we want to look at the system... Am I wrong?