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# Breaking wood

## Intro

Characteristics that make wood such good material to use in breaking.

## Characteristics

If trees were rigid, unmoving objects, they would snap in high winds. However, the composition of wood allows trees to flex with the wind, absorbing the force of the wind without breaking, at least until their flex limit is exceeded, and then the trees break.

When a single board is struck, it bends like a bow, until the flex limit is exceeded and the internal fibers in the board begin to tear apart. When a board is flexed, the bottom surface must stretch more than the upper because it moves in a larger arc of a circle. This means that the bottom will start to crack first. Once cracked, the wood becomes very weak along the inside of the crack and the crack immediately starts to deepen into the wood. The board tears open from the bottom to the top.

When more than one board is used, the bottom board feels more stress than the other boards since there is no board beneath it to resist its flexing. When the bottom board breaks, the break propagates through each board from the bottom up, breaking each board in turn. Since the stress needed to break two boards is greater than the stress to break one board, when the bottom board breaks there is suddenly much larger stress on the top board than is necessary to break it, so it breaks quickly.

If the strike is too weak to break the boards, they will only bend, storing all the energy in their fibers. Then the boards will transfer that energy back into your hand or foot, and pass it along through the joints, causing pain or injury. Make sure to use enough arm or leg motion to force the boards past their maximum flexing point or all your punching energy must be absorbed by your body.

When the board is struck, it begins to vibrate sinusoidally with a certain frequency and amplitude. The frequency of vibration is represented by f=1/2Π√k/m where k is the spring constant for the board (in this case 274 lbs./in. ) and m is the apparent mass of the part of the board that is moving (0.5 lb. mass or 0.22 kg). This gives a frequency of vibration of about 54 Hz a note that is at the lowest level of our hearing to hear, the sound you hear is that of the board breaking apart.

The amplitude of the vibration is related to frequency times the velocity of the striking. This is represented by A=v √m/k where v is the velocity imparted to the board at contact. The breaking speed v in our example would be 4 m/s or 14.8 km/hr. Although this seems low, for an inelastic collision, the speed of your fist must be twice this speed or 29.6 km/hr. To attain this speed, the arm (0.6 m in length) must snap forward in less than 0.3 s.

An elastic collision is almost twice as efficient, which means that hitting the board with a fore fist punch takes less power than with a hammer fist. However, because a board hits back with the same force with which it is hit, you must be careful when hitting objects harder than your fist. For these objects, hitting with a softer part of the hand, such as with a hammer fist or knife hand strike, will be less painful.

As you double or triple the number of boards, you must similarly double or triple the striking speed. To break three boards, it takes a breaking speed of about 90 km/hr.

## Grain alignment

Grain alignment can affect the success of a board break. There are two basic grain alignments of a board:

### Alignment 1

This a view of the end view of a board cut from the center of a tree, where the grains are such that one face is the mirror image of the other. The amount of force needed to break the board will be the same no matter which side is struck.

### Alignment 2

This a view of the end view of a board cut slightly off-center from the center of a tree. The left side was closer to the center of the tree than the right side so the grain alignment is not symmetrical,

Striking the left face of the board will cause the board easier to break than striking the right face of the board. When held on the edges by a board holder and the board is struck, three forces act on the board: two forces from the board holder who is pushing back against the board edges and one from the board breaker who is pushing against the board center. When struck, the front side is compressed and the board bends and the backside begins to stretch and split.

The grain on the left side of the board is closer together and is more easily compressed. The grain on the right side is wider and thus easier to stretch. Therefore, if the board was struck from the left side it would be easier to break than if it was struck from the right side.

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