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How to calculate the force for splitting a rock/boulder in two?

I did a basic calc based on fragmentation of rock from the wiki calculation pages but I'm not confident in it. Essentially what I did was:

Get the radius/diameter of rock, the rock in question looked sphere-ish so I calculated the volume as 4πr, and then multiplied the volume to 8 joules/cm^3 to get the amount of energy put into the strike or:

AP = 4πr * 8 j/cm^3

The 8 joules per cubic centimeter was taken from the VSBattles Table of Destructive Values for the Fragmentation of Rock. I wasn't sure if it was right since splitting in two might be too weak for fragmenting.
You used the full volume of the sphere when instead you should be using the volume of the actual area destroyed.

When it comes to round boulders, here's how it goes.

The radius stays the same, but the thickness is different. Here's how it should be done.

Area destroyed when cracking a boulder in half= pi*radius^2*height, where height is the thickness of the blade or blade-like object you're smacking it with.

The rock diameter you got is 119.3 cm, so 59.65 cm is the radius. The thickness is the bokken's thickness of its point of impact, which you found out to be 0.31358 cm.

pi*59.65^2*0.31358= 3505.25 cm^3. This is the volume destroyed.

So, rock having a frag energy of 8, the calc is thus done as Volume*Destruction energy (Destruction energy being in joule/cc or centimeter cubed, frag energy of rock is 8 j/cc)

3505.25*8= 28042 J (Baseline Wall level)

Now this took two hits, so I'll divide the result by two.

28042/2= 14021 (Street level+)

That being said however, in this specific case, I genuinely doubt that the usage of the bokken's incredibly-thin point of impact is the correct way to go, given that bokkens don't have razor-sharp edges because of which it'd get stuck mid-way (In Yoshioka's case that ain't a problem but still the bokken does have to go all the way through and the rock will have to give way to the thicker parts of the bokken), not to mention the fact that the rock split's thickness is visibly wider, and there are large fragments falling off from the middle, plus, the cut isn't a perfect straight line. I think you should use the full thickness of the bokken instead, as Yoshioka would have to strike in such a way that there is enough space for the bokken to go through completely without getting stuck midway or running out of room to move.
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I'm actually debating whether it was really cutting or fragmentation
Fragmentation, large pieces are shown to be cracked off of it. With a real katana blade, it'd be a clean cut without any jagged edges and the like.

It'd qualify for cutting only if the feat involved an actual katana with a metal blade.

In any case, I'd suggest you use the full thickness of the bokken instead of its point of impact's thickness, as the cracks in the rock would need to be wide enough to fully accomodate for the bokken's thickness and not just its point of impact, as it isn't as sharp as an actual blade. Supporting this is the fact that the split itself on the boulder is shown to be considerably wide in the scans.
The full thickness of the bokken (about 1 inch) would probably bump it up to 5,000,000 joules using the current calc, I'll add another calc to get the possible value on it and use yours to see which one is proper
The full thickness of the bokken (about 1 inch) would probably bump it up to 5,000,000 joules using the current calc, I'll add another calc to get the possible value on it
Assuming bokken 1 inch thickness (2.54 cm) of the cylinder, the new volume is...

pi*59.65^2*2.54= 28392.5556267 cm^3

Assuming fragmentation (8 J/cc)...

28392.5556267*8= 227,140.4450136 J (Wall level). Divide by two to get 113,570.2225068 J (Wall level)

Assuming compressive strength in your calc (300 J/cc)

28392.5556267*300= 8,517,766.68801 J (Wall level). Divide by two to get 4,258,883.344005 J (Wall level)

So not quite near 5 megajoules, but close, assuming you go with compressive strength.

In any case, just go with the 1 inch thickness end instead of using the impact point thickness (If it were me I'd just remove the impact point thickness and just go with the 1-inch bokken thickness).
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