In this fight (at about the 0:51 mark) Chris Rose mentions that Beta's hammer "swings nine feet in point two seconds". I think that gives me some data for a calc. I'll use Imperial and enter the data into Team Cosmos' "Kinetic Enery" calculator at the end.
Speed
9 ft in 0.2 sec equals 45 ft per sec, or ~30.68 mph. Now, the arc is 9 feet times 2 (18 feet) so that's 2.5 revolutions per second, or 150 RPM.
Mass
Grant Cooper, one of Beta's builders, says their biggest hammer head is 10 kg (22 lbs).
Model
The calculator uses different basic models to find MOI and KE. My options are:
Bar—nope, not a good model for a hammer.
Disc—certainly not.
Cylinder—ditto.
Teeth (all the weight's at the end)—best choice.
Calc
Because the calculator thinks of "teeth" as square-ish, I'm going to cheat and just enter random numbers (except for length, which I'll keep low to keep MOI accurate) in until the weight equals 22 lbs.
Okay, with a length of 3 inches, a width of 10 inches, a thickness of 3 inches, a distance from the center of 30 inches, an RPM of 150, and a density of 1.1111 lbs/in^3, I get a weight of 10 lbs and a/an MOI of 63.124 ft-lb-sec^2. The KE is 328.2 J, which seems low.
And I'll check that with this calculator, remembering that it's in metric. I got 670.8718992967803 J, and I'm sure I messed up somewhere. But now I've got an approximation.
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