Mind. Blown.

"You can tell a Finn but you can't tell him much" (youcantellafinn)
12/30/2016 at 15:04 • Filed to: smarter every day, slow motion

Smarter Every Day has some very interesting videos, but the latest left me watching it slack jawed. I don’t really want to give it away to people before you have a chance to watch the video, so consider this your warning. I’m going to post spoilers to the video as a reply to this thread. I’ll leave you with the advice to watch very carefully what happens when the bullet strikes the Prince Ruperts drop. It is mind blowing.

DISCUSSION (9)

Urambo Tauro > You can tell a Finn but you can't tell him much
12/30/2016 at 15:32

I wonder if dipping the drop in something like Plasti Dip would have an effect?

You can tell a Finn but you can't tell him much > You can tell a Finn but you can't tell him much
12/30/2016 at 15:34

You’ve been warned, there are spoilers in this reply. If you’ve watched the video carefully you may have noticed something quite intriguing happening when the bullet strikes the Prince Ruperts drop. The bullet gets shattered. Absolutely annihilated. When the bullet strikes the head of the Prince Ruperts drop it is reduced to tiny fragments and doesn’t leave so much as a scratch in the glass. There is a small amount of lead transferred, but it doesn’t actually scratch the glass. We can tell it doesn’t scratch the glass because if it did the cracks would start propagating from the site of the scratch and not from the tail.

The glass is destroyed not by the bullet strike, but by the shock wave bending the drop enough to cause cracks to form in the tail that propagate back up to the head of the drop. Since cracks in a solid propagate at the speed of sound in that material we can estimate the speed of sound in glass from the video.

Without knowing the exact ammo being used we can only estimate the muzzle velocity. For a .22 long rifle we’ll assume it’s standard velocity ammo which gives a muzzle velocity of ~400 m/s. After hitting the drop there is one shot with a good side view where the fragments fly off at roughly a 45° angle. That gives a velocity along the drop of around 285 m/s making a bunch of assumptions and doing a back of the envelope calculation. The glass cracking propagates up from the tail in about the time that the bullet fragments have gone maybe 10-15% of the length of the drop. From that we can estimate the speed of sound in the glass to be around 6-10x that of the speed of the bullet fragments. Since we estimated that speed as around 285 m/s we can estimate that the speed of sound in the glass is somewhere in the 1,710-2,850 m/s range. In actuality the speed of sound in glass is in the 3,000-5,000 m/s range. Not a bad estimate considering, but we could do it better by taking a little more care to measure the angle the bullet fragments were flying off and looking more closely at when/where the crack propagation meets the bullet fragments.

If we had some type of scale in the background and a timestamp we would actually be able to measure the muzzle velocity of the bullet and the speed of the crack propagation.

tacogx > You can tell a Finn but you can't tell him much
12/30/2016 at 15:35

woah

You can tell a Finn but you can't tell him much > Urambo Tauro
12/30/2016 at 15:38

You mean to cool it? It depends on the heat capacity of what you are dropping it into. If whatever liquid you drop it in is able to remove heat as quickly as the water it will be roughly the same strength. I suspect Plasti-Dip wouldn’t be able to cool the glass off quickly enough to give it the strength of a Prince Ruperts drop.

The Prince Ruperts drop gets it strength by being cooled off super quickly. This puts the outside in high compression which makes it very difficult for cracks to form as the surface is being squeezed together everywhere. The inside of the drop is in a lot of tension though, which is why a Prince Ruperts drop will shatter so completely if even a small nick forms. You can smash one with a hammer and it will be fine, yet put the tiniest nick in it and it shatters into millions of pieces.

Urambo Tauro > You can tell a Finn but you can't tell him much
12/30/2016 at 15:50

Not what I meant, but that’s an interesting idea, too.

No, I was thinking more about taking a cooled PRD and coating it (or maybe just the tail?) to maybe absorb vibrations. Might have to experiment with different thicknesses, though...

jimz > You can tell a Finn but you can't tell him much
12/30/2016 at 16:37

Note that a Prince Rupert’s drop is tempered/toughened glass, which is what car side windows, backlites, and moonroofs are made from. It’s why when they break they shatter into thousands of little glass pebbles.

And to your other comment, the Slow Mo Guys on YouTube have a video where they shatter a Pyrex pitcher via thermal shock. The handle of it is tempered and they had to use the maximum frame rate the camera was capable of to watch the cracks propagate through it.

jimz > Urambo Tauro
12/30/2016 at 16:39

I think all it would do is hold the outer layer of glass bits together. Tempered glass like this has a tremendous amount of internal forces, and shattering it like that releases far too much energy for a thin layer of rubber to damp much.

You can tell a Finn but you can't tell him much > Urambo Tauro
12/30/2016 at 17:54

Now I see what you’re getting at. Hard to say if that thin of a coating would have enough damping. Maybe if it was the right material, or a thick enough coating it would be able to damp things out enough to keep the stresses from getting high enough to initiate a crack.

Noodles > You can tell a Finn but you can't tell him much
12/30/2016 at 18:01

Great video.