1. The problem statement, all variables and given/known data In Robert Heinlein's The Moon is a Harsh Mistress, the colonial inhabitants of the moon threaten to launch rocks down onto Earth if they are not given independence (or at least representation). Assuming that a gun could launch a rock of mass m at twice the lunar escape speed, calculate the speed of the rock as it enters Earth's atmosphere 2. Relevant equations KE = 0.5mv^2 PE = -G M1 M2 / R 3. The attempt at a solution I found the lunar escape speed to be 2374 meters per second. Beyond this, I can't get any farther. I've tried using the lunar escape speed and initial velocity—4749 m/s—to use conservation of gravitational potential energy, but I'm not getting the correct answer, which is 11.9 km/s. Any help would be appreciated!