|
I am surprised no one has really focused in on the kinetic energy vs potential energy thing yet.
Obviously at the bottom of the hill, you only have kinetic energy. If you were to turn the engine off and coast up the hill, at the top you would have exchanged that kinetic energy for potential energy. One of three things would have occurred:
1) You came to a stop at the top of the hill, That is, the kinetic energy (speed) exactly matched the gain in potential energy (height)
2) You came to a stop BEFORE you reached the top (or you had to turn the engine on to reach the top). That is, the kinetic energy (speed) was not enough or you had to add energy.
3) You reached the top at a reduced speed. That is, the kinetic energy was greater than potential energy needed to reach the top of the hill.
In all three cases, kinetic energy is exchanged for potential energy in exactly the same amounts.
But what isn't accounted for is rolling resistance - which is strictly a function of distance - and aerodynamic losses - which are strictly a function of speed (the square of the speed to be exact!). Since the lowest average speed is when the vehicle comes to a stop BEFORE it reaches the top of the hill, anything else consumes more fuel.
But aerodynamic losses at slow speeds are pretty small - which probably why people are arguing the point (the principle doesn't manifest itself clearly). I think you'll find that it becomes easier to understand if you think about going DOWN the same hill. Clearly, using the brake to limit the speed ALWAYS results in energy loss.
|