1. The problem statement, all variables and given/known data Find the average velocity of shell while its moves a velocity v' and expands a velocity v. Sphere radius is R Expansion velocity v Movement velocity v' 2. Relevant equations I think theres no need an equation. 3. The attempt at a solution I try to find average velocity but its vector.So I made max velocity v+v' and min velocity v-v' then I add them and divide two and I find v but I think I made a mistake.