[Moderator's note: Recategorized thread to "Basic".] While driving alone through the beautiful scenery of Banff and Yoho national parks, a question formed in my mind. Which of these modes of slowing down a vehicle by an equal amount is likely to minimize the resulting overall increase in entropy? I’m assuming that the reduction in kinetic energy can be accomplished in a less or more efficient manner. 1) Applying the brakes. 2) Opening all the windows. 3) Turning on the AC compressor, and letting the turning of the wheels power it. 4) Downshifting into a lower gear, and using engine braking. 5) Applying a rainfall. 6) Applying a magnetic field. Any other methods that can be suggested are welcome, if they make any difference to the outcome. A number of reasonable assumptions would need to be made about the specifics involved, and that’s fine. I realize that there may be too many unknowns and not enough specifics, or that the answer may very well be ‘same’ for all the modes involved. I get a sense that this is either a really easy problem, or a really hard one.