When we studied the conservation of energy as it applied to a roller coaster, we pretty much ignored friction and its effect on the situation. Ignoring the effects of friction allowed us to assume that all of the kinetic energy in the roller coaster would transfer directly to the potential energy and vice versa as the coaster went up and down the hills and through its loops. If we pay attention to the effects of friction, however, the mechanical energy of the system is no longer constant. Some of that energy is lost to the surroundings and serves to increase the internal energy of the coaster itself. We know that the conservation of energy concept is still true, but now we can think about how some of that energy is transferred as heat caused by friction.

So, let's follow a coaster as it starts down its first hill. The potential energy (GPE) at the top of that first hill is at the maximum. As it goes down that first hill, most of that potential energy is transferred to kinetic energy (KE), and some is transferred into heat energy (Q) due to friction (goes both into the surroundings and the car). At the bottom of the first hill, we still have a kinetic energy, but the total energy consists now of heat and kinetic energy (Q + KE). As it goes up the next hill, it will not be able to go as high as the first hill because the car has to do work to overcome the friction of the track, "losing" even more energy to heat. Taking into consideration how friction effects the roller coaster definitely gives us a more accurate picture of what actually happens.