BR Physics III # 17: As a mining travel down the rails of a mine, what is true of its relative energies? Originally, I thought that the PEinitial= PE + KE (at a particular time).. but it turns out that there is friction. Do you always assume that there is friction? Also, it says that "as the cart travels the track, the work down by friction increases." Why does friction increase as you go down the track?