Imagine getting hit in the face with a bag of cement. That's what it was like for Ronda Rousey getting kicked by Holly Holm. (From 2015)
predicting that undefeated MMA fighter Ronda Rousey would beat Holly Holm last weekend—but that's not what happened. Instead Holm won the match with a well-placed kick to the head. Boom, Ronda was out. If you really knew who would win these matches ahead of time, why fight?
From a physics perspective, what can we figure out from this final kick to the head? Can we estimate the impact force from this hit? Well, yes—we can always estimate something, even if it requires some wild guesses. That's never stopped me before; why should it stop me now? Let me start with some video analysis. If you search for a while, you can find a shot of "the kick" seem from above. This view is nice, since you can see the whole horizontal motion of the kick as Holly whacks Ronda in the face. It's not a perfect video , but it will do. Scaling the video based on the length of Holly's leg, I get the following trajectory for her foot.In case you missed the fight , the blue data shows the foot moving from upper right down to the left.
But what about the force? First, let's think about the Momentum Principle. This says that the net force on an object is equal to it's rate of change in momentum, which I can write as: I know the change in velocity for this foot—so if I can get values for the time of impact and the mass of the foot, then I will get a value for the force the head exerts on the foot . Now for my first wild estimate—the mass of the foot is about 1.2 kg based on