Hi, I need help with interesting assigment. So, here it is: I have to work out time t. t is time of a trip by a tunnel digged between LA and NY. Distance between LA and NY by surface is 5000km. Earth radius is 6378km. Friction is ignored. The motion is in a train propeled by a gravitational force Fg. Earth is homogenous. When the train is moveing towards the centre of earth the radius is decreasing R->r and then after the centre is increasing again. Picture for this assigment is here: http://forum.matweb.cz/viewtopic.php?pid=51967#p51967 Thanks a lot.