Answer
Please see the work below.
Work Step by Step
According to Newton's second law of motion, the rate of change of momentum is equal to the net external force acting on the system. Thus to keep the momentum of a system conserved, net external force on the system must be zero.We know that gravity is also an external force, but gravity acts perpendicular to plane of collision and hence the net external force along the plane is zero and momentum is conserved in the plane.
