What is a closed system? This term is used in conservation of momentum. Usually involves more than one body. lets say two bodies collide , the system is considered a closed one if they exert a force on each other , there is not outside /external force on the two bodies. Hence it is a closed system.
If you analyse carefully , weight is a external force on the two bodies . hence the system cannot be taken to be closed system. However since the collision involves a direction perpendicular to the weight direction, hence weight does not have any effect in the direction of motion hence can be considered closed in the horizontal sense for solving problems in the horizontal plane,
