A long back spinning
kick is a kick where the practitioner (me.) spins using the foot that is initially
behind them to kick in a circle by raising their foot at they go. The kick is
intended to hit the target 180° after motion has started.

* Please note this is
actually not a circle it is an oval because I have to shift my weight as I go,
but for this problem we’re going to call it a circle.

I know that I can kick hard enough to
break someone’s ribs. It takes 3300 newton’s to do this (the average martial
arts master can, depending on the kick, get up to 9000 newton’s.) I’m not a
master and I’m pretty small so let’s just stick to breaking a rib here. So
torque is:

T=rFsinΘ

Where Θ is
90. The length of my leg is about .69m and since I’m pivoting in a circle the
distance to my foot is the radius. So the torque is going to be

.69*3300*sin90

Torque is 2277 Nm.

Now to get the Radial Acceleration we use the
equation T=Iα if we assume my leg is a thin straight object with the point that
we rotate around being the end. Than the equation for torque should read. T=[(mL^2)/3]*α.

Now I have a mass of 50 kilograms. (well 49.8 but
let’s call it 50.) And my leg to my hip joint is about .69m. And earlier we found
the torque to be about 2300Nm so we get

α=(T*3)/(ML^2)

α=(2300*3)/(50*.69^2).

α=23.8m/s^2

So to get the angular
velocity of my foot at the point of contact.

α=ω/t.

So I timed myself kicking and it takes
me.75 seconds for a half a circle. Because we assume once I hit the guy I stop
spinning.

ω=αt

23.8*.75=ω

ω=17.9 m/s.

So in order to get the linear velocity my foot
hits the target with we take the angular velocity and times it by the radius.

V=ωr

17.9*.69=V

V=12.35m/s.

To give you a better reference for
this number 12.35m/s is 27.63mph.

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