Collisions and Impact.

Analysing collisions can be rather complex. In this it helps to understand certain basic principles of physics, but it is often hard to correctly apply and deduce anything from a martial perspective on collisions. This difficulty can be attributed to the fact that most physics models assume simple geometric shapes with homogeneous mass densities and even structural strengths. Anyone can look at a human and see that this is not the case; we are made up of soft tissue and bones, muscles, organs... which all have different qualities but yet are all linked together and need to be considered when discussing the effect of collisions and impacts on human bodies.

Some of the physics concepts to consider in collisions are energy, force, momentum, speed, mass, power. Since the human body consists of many moving parts it is often difficult to understand which parts of the body are even involved in strikes.

Speed

Speed is often emphasised in discussions of collisions while often ignoring momentum to the point of writing off the importance of mass in the equation. Yes, a bullet is very lite and will easily pierce a human body. Yes, energy is vitally important in collisions. As anyone familiar with physics will quickly point out: kinetic energy is the square of velocity (times mass). This does imply that increasing speed by a factor of X will result in more energy a system than increasing mass by that same factor. So, if you are going to one piece of the equation, speed will have the greatest affect on the energy of a collision.

Momentum

But what about momentum? Both speed and mass influence momentum linearly. While changing the way that you strike may increase the speed of the mass involved in the collision, it is unlikely to improve this speed by much (say 100%). However, depending on how you are currently striking, the same cannot be said for mass. If you are simply throwing your fist at someone without moving anything beyond your arm, the mass propelled by you is likely not even one fifth the mass of your entire body (rough idea: 2 arms + 2 legs + torso = 5). So, by simply holding your arm in front of you stiffly, you could generate a strike with 5 times the mass than a "no-body" punch. This should easily explain why punching while grappling on you back is not very powerful, you cannot accelerate your center of mass, only that of your arm.

The Steamroller

Let's use an obsurd example, that of a steam roller moving very slowly and colliding with a person standing on a skateboard. This collision is likely to be fairly harmless, would it not? Of course, this changes if the person were bolted to the ground, so what does that mean for us? How important is velocity and how important is mass? Well in a fighting situation I probably do not have my feet bolted to the ground and I probably could "roll" away from the steamroller, as long as it wasn't rolling really fast (but yet still slow compared to a bullet)! So tactically, striking slowly against an opponent which is able to evade is likely useless no matter how much mass is involved in the collision. However, even slow strikes with great momentum can be harmful to an opponent who cannot evade it.

The Bullet

Well, there is a corollary to the steamroller's mass, but for speed, the bullet. Bullets are obviously deadly, certainly due to their speed, but what about energy and momentum? The energy is high but the momentum is actually fairly low. After all, the person firing the gun will receive the same change in (magnitude of) momentum as the person being shot! How can that be, one person is injured or killed while the other merely has their hand slightly recoiled? Another way to see the truth to this is if we protect ourselves with a kevlar chest plate, we can easily absorb the momentum of an oncoming bullet, police officers rely on this all the time. So, momentum might seem important after all and speed may not be the end all be all.

Impact Effects

So what about from just the impact perspective (without considering tactics)? Which is more important/destructive, momentum, speed, energy, force? I think that a lot of the answer relies on what your are hitting. Speed seems to be able to damage surfaces, a bullet will damage the surface of your flesh one layer at a time until it chews its way all the way through you. But if you insert a strong (usually hard) surface in the mix, all layers behind it will probably be OK. On the other hand, collisions involving high momentum may not have enough speed or force to do surface damage, but they may still damage other points of a body/mass. Boxers pound each other around with the goal of knocking each other out. This usually occurs because of the brain being smashed internally against the skull, but yet the skull (the surface) may remain intact. It seems that these types of collisions damage the weakest parts of a body, no matter where in the colliding mass they are. It may be important to point out that in both types of collisions there will be a lot of energy whether it comes from the mass or the speed component.

Some Conclusions

For martial purposes it seems likely that a high energy strike with both speed and mass is important. Speed is important since without speed a strike becomes just a push (which in itself could be very useful), or too simple to evade. But mass is also important since I will never be able to make my hand move as fast as a bullet. Having momentum will even help me strike effectively against padded targets. Speed may help me bruise limbs while mass may help me bruise organs (since they are week and susceptible to damage from a full body acceleration).

Reference: Need to incorporate studies on brain damage for boxers

Still needs to address the mass involved in a collision. Strikes will involve more or less mass from the striker's body in the collision. The conclusion from this analysis will lead to the importance of stepping in strikes. Without stepping, the COM is very limited in its involvement, it can only rotate or move within the confinements of the foot base.

A discussion of the surfaces struck can ensue, how can damage be minimised from a defender's perspective?

A discussion on energy absorption might be valuable. Tie this article in with Bounce

Relate being shot and thrown as a misguided perception of large momentum, to simply being pushed unexpectedly.

Talk about power, i.e. the ability to add energy to a strike, particularly to affect stability (in effect, adding momentum)