Question

1. why are perfectly elastic collisions considered closed systems?

no external factors act on the system, which allows mass and energy to be lost.
there are external forces that act on the collisions, causing energy to be added to the system.
mass is added to the system to keep the energy in balance after the collision.
there are no external forces acting on the collisions, and no energy or mass is lost or gained.

Explanation:

A closed system is defined as one with no external forces acting on it, and no transfer of mass or energy into or out of the system. Perfectly elastic collisions meet this definition because both momentum and kinetic energy are conserved, with no external interference or loss/gain of mass/energy. The other options contain inaccuracies: the first incorrectly states mass/energy are lost, the second mentions external forces adding energy, and the third claims mass is added, all of which contradict the properties of a closed system and perfectly elastic collisions.

Answer:

There are no external forces acting on the collisions, and no energy or mass is lost or gained.