Sunday, May 19, 2013

Energy, momentum, and your fists

Momentum and energy may seem very similar at first, but understanding the differences between the two can help you understand the "why" behind a lot of techniques in martial arts.  It will also help you improve your bullshit detector immensely.

Formal definition:  mv
In English:  Mass times velocity
The special part: It has a specific direction assigned to it

An object with a lot of momentum (like a speeding truck) will have the ability to knock you over.  When it comes to fighting, momentum is all about having the ability to knock things down or push them back.  Here are some useful things to know about momentum when you fight:

  • Mass and velocity are equally as important, so if a 200 pound man can knock you over by jogging into you at 5 mph, a petite 100 pound woman running at 10 mph can knock you back just as far.
  • Momentum only works in one direction, so if someone comes charging at you, and you step to the side, they will lose momentum by changing their direction to tackle you
  • "Push" strikes are high-momentum style strikes.  Some of this momentum comes from your muscles immediately after impact, and some of it comes from the momentum behind your own fists/feet.
  • Your fist weighs close to one pound, but with a lot of training, you may be able to get up to 10% of your body weight into the "effective mass" of your punch, giving it 10 times the momentum.
  • A knockout punch requires a lot of momentum, but momentum by itself is not sufficient.  You also need to be able to translate the linear momentum into rotational momentum about a pivot point at the base of the skull.
  • Boxing gloves increase the momentum of a strike by doubling the mass of your fist, making a knockout punch much easier than with bare knuckles.
  • Neither soft foam nor hard plastic can significantly reduce the transfer of momentum.

Formal definition:  ½mv2
In English:  One half times mass times velocity, times velocity again
The special part: it can be transformed into structural damage at the point of impact

An object with a lot of energy (like a bullet) will break bones and damage tissue.  When it comes to fighting, energy is closely related to breaking bones, dislocating joints, breaking skin, and causing pain. Here are some useful things to know about energy:

  • Mass is somewhat important, but velocity is key.  If you double the mass, you will double the energy.  If you double the velocity, you will quadruple the energy.
  • "Snap" strikes are high energy strikes, where speed is the primary focus.  This kind of strike will not knock out an opponent, and it will not knock an opponent back, but it will cause pain and tissue damage in the immediate vicinity of the impact.
  • Besides energy, there are two other factors you need to consider when talking about pain and/or tissue damage:
    • Surface area - The smaller the surface, the greater the damage.  If a bullet was the size of a softball (with the same mass and energy), it would not be nearly as lethal.
    • Rigidity - The more rigid the target (and the striking appendage), the more pain/damage.  This is why shin-to-shin kicks hurt so much, and why a quick elbow across the forehead is likely to break skin.
  • Boxing gloves and MMA gloves make high-energy strikes nearly impossible by
    • Reducing the speed of the punch by doubling the mass
    • Reducing the rigidity of the impact surface
    • Increasing the surface area of the impact
  • Energy can be easily dispersed by soft foam or hard plastic, which is why many sports helmets feature a combination of both.
In the end, it is all about choices
Now that you understand the differences between energy and momentum, you need to start asking yourself questions.  Fighting is no longer about trying to throw the "best" punch.  You have options, and you should strike according to your own goals.  Do you want to cause pain/bleeding and destroy his will to fight, or do you want to knock him to the ground?  Do you have an opening to land a knockout blow to the chin, or do you have a better shot at a bloody nose?  Strikes are tools to get to the final result, and no one tool fits every situation.

Watch out for bullshit
Unfortunately, everyone from martial arts masters, to doctors, to enthusiastic amateurs will tell you something about the "force of a punch" or the "power" of a punch, or which kind of punch is better.  I honestly believe they have good intentions, but if they don't tell you what the specific goal of the punch is, it's kind of like telling you that a hammer is better than a screwdriver without first telling you if you have screws or nails.

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