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Originally Posted by Vman455
That's one of those things that people "know" intuitively: that coasting downhill makes up for expending energy to get up the hill in the first place. In a simplified, idealized world where cars have no aerodynamic drag, no rolling resistance, and no drivetrain inertia or friction, that would be true. But the world is a lot more complex than we imagine most of the time.
I'm working through White's Fluid Mechanics this summer, and the first chapter starts with a discussion of one deceptively simple question: What is the difference between a solid and a fluid? The author points out that most laypeople, even though they "know" what makes the two states different, can't actually define it.
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Fluids flow and conform to a container, solids (mostly) maintain their shape, barring sufficient outside forces. Why is that difficult?