1. The problem statement, all variables and given/known data A projectile is fired straight away from the moon from a base on the far side of the moon, away from the earth. What is the projectile's escape speed from the earth-moon system? 2. Relevant equations Escape velocity = sqrt[2GM/R] 3. The attempt at a solution What I'm wondering is, do I just have to use this formula the moon or do I have to take the gravitational potential energy of the earth and the projectile into account also?
Certainly. you need an equation for the energy needed to escape from the earth and the moon, and you can add those energies.