I am not sure that this fits into this section of the forum, but its something i was thinking about last night while trying to fall asleep. This is hypothetical and not achieveable with current technologies, but if you were to dig a tunnel straight down so it would cross the exact center of the earth, then you completely removed all particles from the tunnel so its a complete vacumm. Then you proceed to drop a ball with a mass of, say 1 kg down it, would it forever fall from one side of the earth to the other. My reasoning is that at the surface of the earth the ball has a certain G potential relitive to the center of the earth, so when you drop it into the tunnel all of that would be converted to kinetic energy on the journey towards the center, then back into G potential as it reachs the surface again on the opposite side of the earth, then it would journy back to your side and so on. It would never lose any energy and will continue to "fall" though the earth forever. Is there anything that would stop the ball and how does this "perpetual" motion relate to the laws of thermodynamics.