The amount of Physics you need to understand what happens in martial arts is pretty low,
but you won't be able to do techniques as reliably without knowing a bit. This page has
the sorts of concepts I find I refer to all the time in martial arts classes. This is
driven not by some effort to catalog everything, but what I find I need in my specific
martial arts system, hakko denshin ryu aiki jujutsu.
people maintain that mysterious powers are the cause of their abilities. Usually
this just means they can't explain what happens. Of course, being in communion
with higher powers is in any case irrelevant: I've had no succes at using
some great supernatural force, no matter how much a few enthusiasts have coached
me. So even if such a thing were real, it does not apply to me and I have to fall
back on other ways to reliably reproduce the effects I am after.
Here is a thought on the difference between so-called external systems (like karate) and internal systems like jujutsu. External systems tend to try and maximize the acceleration to generate force. Internal systems tend to emphasis root and connection to the floor, in effect allowing you to increase you mass -- your largest effective mass is how much force your legs can exert against the floor. This is not to say that both types don't attempt to juggle the other half of the product. Low stances in karate can help effectively increase mass and "shaking" (aka fa jing in Chinese or hakkei in Japanese) are really aimed at letting you accelerate without losing your root.
A simple machine allows a small force to overcome a larger force.
They are referred to as simple, since they require only a single force. There are
just a few basic simple machines and we use them and their variations all the time.
We start with
First class. The fulcrum lies between the load and the point
of application. It looks like this: L----O-----F A standard textbook example is
the see-saw. When you nod your head, this is a first class lever where the head
is the load, the neck muscles supply the effort and the vertebrae are the fulcrum.
First class lever change the size and direction of the force. If you have the fulcrum
in the right place, you can lift a car by pushing down with one hand. Hip throws
are this type of lever with the load as the lower body and the fulcrum is the thrower's
hip. Here we see why using body weight to initiate the throw is such a grand idea:
With the upper part of the receiver toppling, this effectively uses him to increase
the force on his lower body. Since generally peoples upper body are heavier than
their lower bodies, there is invariably enough force to throw the person. This accounts
for their great power.
Second class. The load lies between the force and the fulcrum:O----L----F
A bottle opener or wheelbarrow are the standard examples. On you, whenever you stand
on tiptoe you are using a second class lever, where the balls of your feet are the
fulcrum, you are the load and the calf muscles supply the force. Second class levers
multiply the force, but do not change its direction. Throws of this sort are shiho
nage, irimi nage, as well as throws where the lower body is blocked, such as
with a foot pin. Most hip throws incorporate this type of levering action initially
to get the opponent off-balance, then switch to another levering action.
Third class. The force is applied between the load and the
fulcrum: O----F----L Tweezers are a standard example, as is swinging a baseball
bat -- or a samurai sword. On you, your forearm works this way, where the
load is the forearm, the fulcrum is the elbow and the force is generated by the
biceps. Third class levers multiply motion but reduce the power. So while it takes
a relatively large force, the load moves much further. Armbars and ankle bites are
examples of this, as are most takedowns like kibusu gaeshi, in which the
leg is fixed at the lower end and force is applied to the knee or hip to effect
the throw -- this is because these throws are variations on armbar-type locking.
Actually most joint locks in our system are wheels. In this case, the axis of
rotation is the through the joint at the base of the thumb. The traditional way of
explaining this is the talk about the power of the pinkie, since the pinkie follows
the rim of this wheel. Consider our basic wrist escape, hakko dori. What
happens at the Physics level is that the attacker is suddenly trying to hold onto
the rim of a powerful wheel. This is not possible (have you ever tried to
grab a spinning wheel?) A lot of the wrist manipulations in the system are devastatingly
clever ways of exploiting the wheel (don't forget there are three dimensions and
switching of directions judiciously has to be done.)
This allows for a rotatry motion to be converted into a linear
motion, such as forward or backwards. A screw is a continuous inclined plane. This
is seen in many motions that involve the concept of maki komi. or wrapping.
Another frequent way is when moving into the twisted stance, kosa dachi,
in which the dropping, turning motion means you are the screw and the action on
your grappled opponent is to pull him powerfully off-balance. Other screwing actions
can occur in joint locks, like te kagami, in which the entire upper body will be
moved by using uke's whole arm as the screw.