alt Hacker NewsOh, 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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The “proportionality constant” is doing a lot of work in that claim. A lot of “constant” parameters are swept under the rug. If you fix enough stuff that claim is indeed correct, although I agree a bit simplistic
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?
Yep, cars can accelerate at over 1g.
Friction is proportional to the normal force, more specifically, it is the normal force times the coefficient of friction.
What you are describing (if the normal force is actually the same) is a contact situation where the coefficient of friction is different in different directions (anisotropic friction.)