An, interesting concept that I heard of was using gravity to crate a perpetual motion machine. So here's how it works, you drill a hole through the earth, equator to equator with both entrances to the tunnel located at exactly sea level. You line the tunnel with some thing that allows you to turn the tunnel into a vacuum chamber.
Then you drop a metal sphere into the tunnel. Now here's my question, would the force of gravity pulling the sphere towards the center of the earth get it moving so quickly that it's momentum would be great enough to get to the other side of the earth and have it stop or slam into the exact same elevation on the opposite side of the world. If it made it through the earth it would oscillate forever, but if it were off at all it would eventually settle in the middle.
I've been trying to think this through but I really can't come to a conclusion. So if anybody out there could find the equation of a falling object in a vacuum and the rate of deceleration due to gravity in a vacuum. Then run the numbers and see what the sphere will do.
