The Physics of a Strike

To understand redirecting and taking the center of mass we need to understand the physics of a strike and why being hit is a bad idea.

There are several aspects that can analyzed with a typical punch-

1. Force (F=ma)
2. Kinetic Energy (K.E. = .5m(v^2))
3. Torque
   

Force-

    When discussing Force in regards to striking the goal of the attacker is to put as much force into his opponent as possible (without    sacrificing the attacker’s balance and coordination). From observing F=ma is it clear to see that the best way to increase the force of strike comes from increasing the acceleration of the strike. The mass of a fighter cannot really change but if the acceleration is raised then so does the force. Bruce Lee is an excellent example of someone who rapidly accelerates his fist when punching.

GIF Credit: https://upload-assets.vice.com/files/2015/07/01/1435767480Bruce_Lee.gif

His fist makes contact with his opponent in almost no time and it is clear how devastating the blow was as his opponent went to the ground quickly.

Kinetic Energy-

      The goal of the punch is to deal as much damage to the opponent as possible and so the attacker wants to transfer as much energy as possible into their  opponent.

As seen by the equation for kinetic energy-

K.E. = (.5)(m)(v^2)

The velocity of the strike actually will contribute most to the energy because it is squared and while the mass of the arm or body can be different (such as the difference between a lightweight and a heavyweight fighter), the mass is not squared in this equation which seems to imply that the velocity of the strike has a bigger impact on the kinetic energy.

 

Gif credit: https://s-media-cache-ak0.pinimg.com/originals/ca/d0/d4/cad0d417ad515d97a332b21b4434f0ac.gif
Notice how quickly Bruce is delivering his strikes, this is to maximize the kinetic energy. A quick calculation with reasonable numbers can give a rough idea of how much Kinetic energy is in these strikes.

If Bruce Lee has a arm with a mass of 3.5 Kg (This is just a guess based off his body weight and the average arm to body mass ratio)
and he punches at 20m/s (this is based off the fact that one of the fastest punches recorded recently came from a boxer who punched at 15m/s and Bruce Lee was considered the fastest)

His Kinetic Energy would be (0.5)(3.5Kg)(20^2) = 700 Joules which is about 515 foot pounds. This is extremely powerful which explains why some who were punched by Lee would be hospitalized. 

Torque-

Many fighters recognize that the arm alone cannot generate enough force to fell their opponent and so over time the technique has evolved to include rotational energy. When analyzing clips of fighters it is easy to see them twist their feet into the ground and then when their feet do not slip due to static friction that rotation transfers up into their hip and then their torso and finally their arm.

Photo Credit: https://img.washingtonpost.com/news/morning-mix/wp-content/uploads/sites/21/2014/05/Bruce-Lee.gif
It is plain to see that his feet begin moving first and then observing the flow of energy it ends with it being expelled out of his fist. This torque is also applied when the attacker will twist their fist from the knuckles facing upward at first to then facing downward.