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Electron band structure in germanium, my ass (2001)

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903 points tux3 | 1 comments | 01 Apr 25 12:25 UTC | HN request time: 0.208s | source
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BeetleB ◴[01 Apr 25 15:49 UTC] No.43548232[source]▶
Oh, BTW, the whole "Friction is directly proportional to the normal force": My Ass!

I could never reproduce it well in the lab, because it's really not true. Take a heavy cube the shape of a book. Orient it so that the spine is on the floor. It's a lot more friction to move it in one direction than in the transverse direction. Yet the normal force is the same. Any kid knows this, and I feel dumb it never occurred to me till someone pointed it out to me.

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mizzao ◴[01 Apr 25 15:59 UTC] No.43548343[source]▶
Is this possibly because you need to use additional force to horizontally stabilize it in one direction (perpendicular to the spine) but not the other?
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1. mercutio2 ◴[01 Apr 25 16:55 UTC] No.43548995[source]▶
I was about to say exactly this.

Applying force directly to the center of gravity with one finger is hard.

You end up applying torque plus adjustments in response to that torque. And that is heavily dependent on your moment of inertia, unlike the normal force.

But I do agree that explanations of friction are right up there with “how do airfoils work” where poor instructors are liable to get long past the edge of their knowledge and just make shit up.