Q: The residents of a small planet have bored a hole straight through its center as part of a communications system. The hole has been filled with a tube and the air has been pumped out of the tube to virtually eliminate friction. Messages are passed back and forth by dropping packets through the tube. The planet has a density of 3640 kg/m3, and it has a radius R=5.39×106 m. What is the speed of the message packet as it passes a point a distance 0.460R from the center of the planet? I can find the mass of the planet, but other than that I'm just stuck.