Consider the following scenario. A car for which friction is not negligible accelerates from rest down a hill, running out of gasoline after a short distance. The driver lets the car coast farther down the hill, then up and over a small crest. He then coasts down that hill into a gas station, where he brakes to a stop and fills the tank with gasoline. Identify the forms of energy the car has, and how they are changed and transferred in this series of events. (See Figure 7.34.)
Figure 7.34 A car experiencing non-negligible friction coasts down a hill, over a small crest, then downhill again, and comes to a stop at a gas station.
Step 1 of 2:-
In the very beginning, when the car was at the top and had not started moving, it has only gravitational potential energy.
As it started moving downwards, the gravitational potential energy gradually gets converted into it’s kinetic energy.
There is a loss of energy due to friction between the tyre and the surface, so that the mechanical energy is not conserved.
But the total energy is still conserved. If the heat generated due to friction also we will take into account, the total energy is unchanged.
Then it comes near a crest, which is like a potential hill.