Two pucks are about to collide on ice. You want to promise, before the data arrive, that the system's momentum will be conserved. The law makes that promise only under its condition, so the condition is what you check.
The check: take the system, inventory the external forces, and ask whether their net is roughly zero. Internal forces never matter for the total.
The system is two pucks on smooth ice.
External forces on the pucks: the Earth's gravity acts downward on each puck, and the ice supports each puck with an equal upward force. Those cancel. Friction from the smooth ice is tiny.
The net external force is roughly zero. Is momentum conserved here? Yes.
The same two pucks, now sliding on rough carpet. Gravity and the upward support still cancel, but the carpet's friction on each puck is large and unbalanced. Is momentum conserved here? No: while that friction acts, the total momentum changes.
The same two pucks, back on the ice, with a steady wind now driving along the surface. The wind's force on the pucks is external and unbalanced. Is momentum conserved here? No.
Two gliders collide on an air track. The air cushion supports the gliders, gravity acts downward on them, and the track's friction is negligible. Decide whether the system's momentum is conserved through the collision.
Gravity on the gliders and the cushion's upward support cancel.
Friction is negligible, so no unbalanced external force acts on the gliders.
The net external force on the two-glider system is roughly zero.
The net external force is roughly zero, so the system's momentum is conserved.
Two curling stones collide far out on smooth ice; friction is tiny and the vertical forces on them cancel. Is the two-stone system's momentum conserved through the collision?
Two toy wagons collide in deep gravel, which exerts a large friction on both. Is the two-wagon system's momentum conserved while they grind to a stop?
Jorge says: the Earth's gravity on two colliding pucks is an external force, but the ice supports the pucks with an equal upward force, so the net external force is roughly zero and the system's momentum is conserved. Is he correct?
Two ice skaters collide and hold on to each other on a rink; friction from the ice is tiny and the vertical forces on them cancel. What happens to the two-skater system's total momentum through the collision?
Two boxes sliding the same way collide on a rough warehouse floor and keep sliding the same way, and the floor's friction on them is large while they slide. What happens to the two-box system's total momentum while the friction acts?
Which question decides whether a system's total momentum is conserved through an interaction?