The conservation of $p$ (momentum) is most closely related to which fundamental principle in physics?

In a closed system, when two objects collide, which physical quantity is always conserved?

Which equation best describes the law of conservation of momentum?

In the context of the conservation of momentum, what is true about the total momentum $p$ of a closed system (one with no external forces acting on it)?

According to the law of conservation of momentum, what must the total $p$ (momentum) of the system be after two train cars collide in the absence of external forces?

In projectile motion, is there any point along the trajectory where the velocity vector $\overrightarrow{v}$ and the acceleration vector $\overrightarrow{a}$ are parallel to each other? If so, where does this occur?