Saturday, January 15, 2011

Quivering blobs, "captured newtons"

This is a remarkably ignorant video; as Flowing Data diplomatically puts it, "I suspect the creators behind the video [G.E.] didn't have a complete understanding behind the math and mechanics." It's not just the obvious howlers -- the nonsensical talk about "total captured newtons," as if forces were additive scalar things you could store -- but the fact that the video seems quite unrelated to its ostensible topic of "dynamic braking." But it's pretty:

For the record, dynamic braking (see Wikipedia) is when the inertial rolling of a train's wheels is used to generate electricity -- analogous to the motion of turbines. [Recall: a motor run backwards is a generator.] Because electrical energy is generated, the kinetic energy of the moving train must decrease and the train must slow down; however, the electrical energy can be stored or dissipated as heat in some other part of the train, whereas a straightforward frictional mechanism would turn most of the original kinetic energy to heat in the wheels and braking system. A mechanical analogy would be a "one-way" bowstring or trampoline that'd stay stretched until you pushed a button to release it... the blobs in these videos aren't anything of the sort, they're merely shock absorbers.

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