this post was submitted on 15 Nov 2024
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[Thread] Mental Math (mander.xyz)
submitted 2 days ago by [email protected] to c/[email protected]
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[–] [email protected] 99 points 2 days ago (5 children)

When sharpening knives, with practice you can tell when you are done by sliding your fingertips along (not across) the sharpened bevel. It's possible to feel imperfections measured in micrometers this way.

[–] [email protected] 77 points 2 days ago (1 children)

If the earth were shrank down to the size of a golf ball, you could feel houses.

[–] [email protected] 148 points 2 days ago (2 children)

That seems wildly unnecessary. I can already feel houses.

[–] [email protected] 2 points 1 day ago

No, you feel a house. Think of how many houses you could feel at once #shrinkearthtoagolfball

[–] [email protected] 3 points 2 days ago (2 children)

I'm sure there's some actual science thing they're referencing here, but I don't understand it. Who can't feel houses?

[–] [email protected] 17 points 2 days ago (1 children)

Both a house and a golf ball are unbelievably tiny compared to the size of the Earth. The idea that you can shrink the Earth many thousands of times over and be able to feel a house parallels with our ability to feel micro imperfections on knife blades.

[–] [email protected] 8 points 1 day ago (1 children)

Oh! As in, should you be an outside observer, and grabbing the whole planet, you'd be able to feel houses on its surface if it were the size of a golf ball?

Presumably you'd feel like crushing under your fingers like the hairs on a kiwi lol

[–] [email protected] 2 points 1 day ago

It would be a lot more smoother, smoother than a marble. But thoose detection of micro imperfections might be possible

[–] [email protected] 13 points 2 days ago

People without fingers, I guess

[–] [email protected] 25 points 2 days ago (1 children)

I mean, most people do it across, rather than along the blade, what with the necessity of detecting a burr, which can't usually be felt length wise. You slide along the blade, and it is sharp, if you screw up you get cut.

That doesn't take away from what you're saying, it's very true, no matter which direction you're feeling. Just normal, average fingertips can pick up stuff like that, that you'd need a microscope to see. It's a trip!

[–] [email protected] 11 points 2 days ago (1 children)

The burr is also detectable lengthwise. When starting with a dull blade it feels smooth while sliding fingers lenghtwise. When the burr is formed, it starts to feel rough. When it feels like it's digging into skin, it's sharp. It's a very subjective thing though, everybody has different fingers.

[–] [email protected] 4 points 2 days ago (1 children)

Is way better to draw your finger perpendicular to the edge to feel the burr. That way you don’t cut yourself on the edge or the burr itself.

[–] [email protected] 1 points 1 day ago

This is how I was taught as a kid.

[–] [email protected] 3 points 1 day ago

Worked at a machine shop for a while, it's funny how the easiest way to gauge surface finish is to run your nail trough it

At one of the places we worked at we would know when the rough cut was deep enough just by feel

[–] [email protected] 21 points 2 days ago

We have equipment to measure down to microns, and my students often test how fine details they can feel.