To make a science-fiction movie reasonably realistic from a scientific perspective, a screenwriter is seeking your advice on one of the scenes. In the scene, one of the actors stand motionless before firing a projectile weapon at a massive alien creature. Compare the total momentum of the actor and projectile before the projectile is fired with their total momentum just after the projectile is fired

 p₀ = 0 , 0 = pactor + p_proyectal

Explanation:

The total moment of the actor and projectile before the shot is zero since both are at zero speed

         p₀ = 0

after the shot the moment is

        = P_actor + P_proyecta

        P₀ = p_{f}

system is formed by the actor plus the projectile

        0 = pactor + p_proyectal

       p_proyectil = - p _actor

See fly momentum before and pulse shot is zero.

In the part after the shot the moment of the two parts is different from zero, but since they go in the opposite direction their sum gives zero,

i think it would be 5

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.