THE PHYSICS OF STRIKING
When you strike an object or person (either by punching or kicking) what exactly are the factors controlling how much force that strike will have? The physics of this process are well understood and agree perfectly with the techniques used and recommended by martial art instructors in Tae Kwon-Do and Karate. The basic technique is :
With the least possible muscle tension, accelerate the striking surface
towards the target as quickly as possible.
Just before impact, tense all the muscles concerned with the body
framework so that as the impact happens the body is as rigid as possible.
is the average force applied during the time interval
is the time interval
is the mass
is the difference in velocity before and after the time interval
When the fist (of mass M) collides
with a target that cannot move away quickly (either because it is very heavy
like a person or because people are supporting it with their bodies), that
object must decelerate the fist from whatever velocity it had before it came
into contact with the target (say V) to rest in a very short time (dt).
To do this it must provide a force (F) which is equal to (M x V) / dt .
If the target cannot provide this force, the fist will continue on
until F x dt finally adds up to M x V some time later.
In the case of wooden boards this 'continuing on' simply bends them.
If they have to bend too far however, the back surface (which is then
under tension) will split and the board will break.
In the case of person's body, this 'continuing on' will probably cause
internal damage as organs are forced to move out of the path of the fist.
If the target was a 40 ton block of steel however, it would have no
problem providing the necessary stopping force instantly and by Newton 3 that
force would have only the contact points on the fist to act on.
This would probably result in damage to the fist.
Mass of the attacking object.
Velocity (speed) of the attacking object on impact.
The amount of time the object takes to stop the attacking object from
most cases this can be measured as a distance before damage occurs.
If the fist, forearm, shoulder etc.
were made from a solid material such as metal and welded together, the mass of
the striking object would simply be the masses of each of these parts added
together. If however, these parts
of the body were only connected by string or wires, the mass of the striking
object would be that of the fist alone. In
reality, the body is connected by bones, joints and muscles which is a
situation between the two extremes. When
the muscles are tensed, the body acts more like one solid object. When
the muscles are relaxed, the body acts more like separate parts.
What happens in a correct punch is this :
If the tension is applied to early,
the effective mass of the fist rises early which reduces the acceleration and,
thus, final speed on impact. If the
tension arrives too late, the effective mass of the fist is low and it will have
less effect. To achieve the most
efficient timing, strength, tension and posture to exploit this technique (which
only lasts a fraction of a second) is not something that is learnt easily.
Without the evidence of destruction demonstrations, the fact that it
could be learnt at all would, I think, be in doubt.
Some other strikes used in martial
arts exploit the two other factors - speed and impact time - more than the
'strongest' attacks such as the side kick and forward punch which rely on the
above technique very strongly. Extra
speed on impact can be achieved in some cases by extending the limbs in rotation
much like a sling-shot or golf swing. Other
strikes exploit the very short time of impact caused by hitting with a part of
the body which is very rigid such as the elbow or head.
`THE PHYSICS OF STRIKING' Written by John Davies for SEVERN Tae Kwon-Do