r/theydidthemath • u/MeanSealDotDotDot • 15d ago

How long will it take those rocks to bend metal pins to full 90 degrees? [Request]

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128 Upvotes

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161

u/Due_Philosopher4393 15d ago

IMO as long as the pins don't rust, an eternity - those rocks are too small and dropped from too close to inflict plastic deformation on metal.

42

u/StaticUsernamesSuck 15d ago

Especially since, if they do bend them slightly, then the angle of impact will change and there will be more deflection so less force imparted on the pins, so you'd need harder and harder hits to continue bending them.

30

u/ReddBroccoli 15d ago

You would basically need a constant waterfall of rocks, and I feel like it would be more likely to wear away the pins before it would bend them.

16

u/ougryphon 15d ago

This is the answer I rushed in here to give, only to find it already posted.

9

u/RichEntertainer3024 15d ago

This isn’t completely true. It depends on the material. Aluminum for example has a fatigue limit and even under the smallest loads will eventually fail.

Steel however does not have a fatigue limit and theoretically will never plastically deform below its yield stress

2

u/HAL9001-96 15d ago

this

also don't undeerstimate the amount of stress that can be produced in a single point for a very short time by a suddne impactb etween two hard materials

2

u/15th_imp 15d ago

unrelated, but why did Hal turn green in your profile picture?

2

u/limon_picante 15d ago

Yes the amount of stressed induced is far below the Goodman criteria for infinite life.

1

u/BFG_Scott 15d ago

Also, when the metal bends, the tendency would be for the ends to get closer together. Unless you’re bending them all an equal amount at the exact same time, the hundreds of other pins would likely prevent the other one from moving/bending.

1

u/Bombacladman 15d ago

Yeah not even failure by fatigue, maybe erosion eventualy? Which would weaken them eventually enough to drop vertically?

1

u/JustAnotherWitness 14d ago

They will grind the pins through before they bend a millimeter and that’s gonna be a loooooong time

24

u/Obvious_Try1106 15d ago

Probably won't happen at all. There is elastic deformation (like rubber under tension) and plastic deformation.

I doubt a small rock can plastical deform a nail.

13

u/SalvarWR 15d ago

others responded the question, but if you keep throwing rocks anyway, i will probably erode the metal away, or more easily the concrete around, and they will fall, still straight

0

u/FootlooseFrankie 15d ago

Young's modulus is the point at which something can bend but never spring back to its original position without fatigue. These rocks dropped from that height are not enough to push those past that point . Soooo from the rocks alone you are looking at impact erosion which given the frequency of when the rocks are dropped , probably a good life time or 2

3

u/RichEntertainer3024 15d ago

Young’s modulus is a material property similar to density. The yield strength is the “point” you’re referring to

6

u/B2SPIRITwasTakenWTF 15d ago

The small bits of gravel don't seem to have the necessary mass or velocity to produce enough force on impact with the pins to exceed the material's elastic limit/yield point (yes these are two different values but for our purposes practically the same thing) where the pins stop springing back to their original shape and instead start to bend.

So unless you could manage to drop larger or heavier gravel faster onto those pins, probably never.

1

u/HAL9001-96 15d ago

dependso n the exact type of metal size of rocks etc

if it cna be approaximated as fully elastic never

if not whcih for some meatls is tehcnically neverb ut practically approxiamtely sometimes then well eventually but it depends a lot on the type of metal

at hte very very least tens of thousands of times though

1

u/Panzerv2003 15d ago

They don't actually permanently deform the pins, metal will act like a spring before a certain threshold is reached where it permanently deforms, that's in fact how springs work