**Concepts of Physics - The Karate blow**

I leave a small pearl that I began to analyze with the help of my friend Sensei M. Fabian King (3rd Dan Karate) who always contributed to the progress of my knowledge.

**1 - Introduction**

In recent years, the ancient Oriental art of karate do (the word literally translated means "the way of the empty hand") has become very popular in the Western world.

The Karateka shows the result part of your workout by breaking wooden boards, cement blocks and other objects. We talked a lot about the history and culture of the martial arts, but this time we will study a little physics, we will study the mechanics of the collision of a hand against a solid object.

**2 - Concept of Force, Moment and Strain Energy.**

In the case of a crash, is well known that a large object moving at high speed, hit harder than a small object moving slower. In an attempt to break a table, the karateka will look to its heaviest blow, this implies that seek to ensure that your hand reaches the highest speed possible ... now what does it mean to strike harder? ... In terms of physics, we can analyze it in two ways, from the concept of force and moment and from the concept of strain energy.

Force (F) is mass (m) times acceleration (a): F = ma

Time (p) is mass times velocity (v): p = mv

acceleration measures change speed in time, ie acceleration is the derivative of velocity with respect to time, therefore, if we replace a by dv / dt and apply resulting in p / dv, we see that F = dp / dt, then we say that F.dt = dp, ie Ft = I (Pulse, or the variation of the moment).

say that the impulse is an amount "preserved" is not created nor destroyed, but in the present case, is transferred from one object to another (along the solid object it hits.) This follows from Newton's third law or principle of action and reaction (For every force acting on a body, it performs a force equal but opposite to the body that produced it). Thus, the second object (beaten) is exactly the same moment that the first object loses.

Assuming that, I is constant, is transferred from hand to object beaten, then F t are inversely proportional. So we I convey the same by applying a strong force in a very small range of time or by applying a small force over a large interval of time.

The next question is ... why then is it necessary to speed the blow? For if the hand moves faster, in theory, when it hits the object, and complying with the principle of action and reaction, slow down fast too, which means that the time interval in which transfer their momentum to the object will be very short, therefore, the force becomes larger. Big

This transfer of force causes a relative acceleration of the hit of the object relative to other parts of the object. Ie the small area struck by the knuckles or the edge of the hand tend to shoot out to beat the same direction as the rest of the board receives no pulse and are also supported by some kind of "fixed." So it is breaking.

analyze this from the concept of strain energy.

As an object of mass m1 (in order to hit), at rest, and another object with mass m2 (the karateka's hand) moving at velocity v at the time of impact, if we ignore the peeling thermal energy in motion, the amount of strain energy would be:

**ΔE = ((1-e ^ 2)) / 2. (m1.m2) / ((m1 + m2)). v ^ 2**

where e is the coefficient of restitution, which measures how elastic the shock. Is a function of the hardness of objects, and which together with the speed, determine the pulse. If two objects collide perfectly inelastic, e = 0, the transfer of force between the two is immediate (short time). If perfectly soft objects colliding (and approximately 1), the energy transferred will tend to 0, or rather, the time interval during which transfers the force becomes larger.

An easier to understand this would be the following, we compare the time of contact and transfer of force from a blow applied to a pillow the edge of the hand and a blow applied to a brick with the knuckles.

As ΔE is proportional to the square of the speed, the faster you move your hand, the more energy transferred to the object hit. Again the end of this story is breaking the object.

**3 - The area hit.**

Anyone who is beaten by hand an object using a wrong technique can attest to the pain it causes. The hand is a complex system of bones connected by tissue, and much could be said about the importance of proper alignment of the hand to strike. From the point of view of physics, the important thing at the time of impact is that all the above in terms of force, momentum, momentum and energy, presupposes a certain unit area.

Reducing the impact surface, the karateka apply all that energy into a small point, ie, the object must be beaten "resist" or suffer a huge amount of energy in a very small point.

practical example: Consider a person capable of applying a shock equivalent to 190 joules (J) of energy. A normal hand is about 15cm long by 10 wide, ie 150 cm2, or about 1.3 J per cm2. Karateka

If only hit the side of the hand (5cm by 3cm), apply about 13 J per cm 2 ... 10 times more energy!! Hence the search karateka Always apply the strokes using small areas of your body as a point of contact (knuckles, elbows, etc).

**4-Focus**

Many times, when he advises a novice how to break a table, you are instructed not to seek to break the table, but look floor breaking wave wall behind her. This is to ensure that the hand is not slowing down at the moment of impact, a mistake that many beginners make for fear of hurting or uncertainty. Said that it is very important to achieve the highest possible speed at the time of impact, and there are several video documents showing that the speed is achieved when the arm is extended by 75%. This is logical if we think the other 25% of the route will be used to stop the hand. It goes without saying that the point of impact is at the time you get to 75% of travel.

This means that to achieve a successful break, we should focus on a target located 25% of the total length of our arm, behind the object to hit. That should be our mind and ki to strike.

**5 - Using the body weight**

see that the mass m is an important factor in the formula of Time, Power and Energy. In all cases, the proportionality is direct.

As the mass of a karateka will be the same before and after a stroke, many times, it disregards the mass in these equations, however, is very important how that mass is used at the time of the coup, much of that mass is involved at the time of hitting. That is, we can not control 100% of your weight and manage to strike at will, but we can control a certain percentage of our weight and apply at the point of impact.

For this reason it is crucial not only to use the arm or leg to hit the whole body should be aligned and "push" in the coup. So often happens that a boxer is knocked unconscious after a hook up (moving from the waist and legs) and is less common but be knocked out by a jab. Although both blows are applied by the same arm with the same force, but in the first case, a large percentage of the weight was applied in the art.

Source: JON Chanani. - Physics of Strikes

University of Virginia, Charlottesville, VA 22903

adapted, interpreted and modified by Paul Cirigliano.

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