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Breaking a Karate Striking Board with Wrecking Ball

  1. Oct 14, 2008 #1
    In my A level physics class i have decided to find the amount of force required to break a karate striking board and relate it to how much force is required to break a human rib and a jaw etc

    The striking board is basically a board that has been cut in half so there is a snapping line across it. but not with a straight cut. looking straight at its flat surface (where you would strike) the cut across the middle is straight. but looking at the side edge reveals that the cut is a wiggly jigsaw type of joint that makes it very hard to seperate from hitting the flat surface. however if you hold the bottom half steady and push on the side edge of the top half it will slide off. this sliding ability means that i can adjust the board to be joined at half a board only or any fraction.

    please ask me if this is unclear as it is very hard to explain

    normally one person would grip the bottom half at the bottom edge and another person would grip the top half at the top edge, so the snapping line runs horizontally. then it would be struck near the centre of the board and it would snap leaving 2 people with half a board each.

    I plan to swing an iron wrecking ball into it so this method didnt seem safe to my physics teacher. Instead i am having to clamp the bottom half into a workbench and swing the wrecking ball into the top half to snap it off. after the joint has 'unclicked' and the boards are seperated they can be slotted back together.

    my question is how will the force required to snap the top half off be different to having it clamped at both edges and something snapping through the middle? As i can not think of any safe way to recreate the board being held at 2 edges and the ball striking through the middle i am forced to clamp the bottom half and snap the top half off. so can my results of froce required for snapping off the top half reveal the force required to break through the middle?
    its a tricky question but im sure you guys can help me.
  2. jcsd
  3. Oct 14, 2008 #2

    Ben Niehoff

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    Why not instead do this:

    Build some apparatus that can hold the board horizontally by the edges. Then stack weights on top of the board until it snaps. Repeat with different boards and finer weight increments to get an accurate idea.

    By the way, the breaking point of the board is actually related to pressure, not force, and so you will have to pay attention to the area of the weights resting on the board. The location the pressure is applied also makes a difference.

    Also, most karate boards I've seen are not made of two pieces of wood jointed together...usually they are single pieces of pine, with the grain running horizontally. My guess is that your jointed boards are supposed to be reusable? But they will also require less force to break than a solid piece of wood.
  4. Oct 15, 2008 #3
    hey cheers for your responce but i was led to beleive that to work out 'how hard the board needs to be hit' i would have to find the sheer breaking force. this would be determinned by hitting it and not by adding weights until it breaks.

    the boards are not actually wood as i stated but a very very hard and stiff thick plastic. it is 2 boards seperated and not 1. they slot together with a joint like a puzzle piece along the edges. and so are reusable.

    so, considering i am right, i still need to know if i can find the force required of a strike to break the board if struck at the centre. using my results of the force required to snap off the top half.

    please answer me because i have scoured every information source i know and could find nothing.
  5. Oct 16, 2008 #4

    Ben Niehoff

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    Like I said, you need to find pressure, not force.

    If you want to compare your results to the force of a karate strike, then you need to apply the pressure using a cross-sectional area equal to the size of your knuckles.

    Good luck.
  6. Oct 16, 2008 #5
    thanks for your responce but i still have my initial problem.

    after finding the pressure required to break the joint in the board by holding the bottom and striking the top i have no way to relate this to how much pressure is required to break the joint if i strike it in the middle

    any ideas?
  7. Oct 16, 2008 #6

    Ben Niehoff

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    Why in the world would you want to test it that way, then?

    It makes sense to build an apparatus that holds the board by both ends, whether it be horizontal or vertical.
  8. Oct 17, 2008 #7
    because there was no way of testing it that way.

    we tried several systems using clamps etc but our apparatus was not stable. So the only way that i could hold it and hit it was by clamping the bottom half in the technology departments workbench and striking the top off.

    So I still have my original problem of how i can find the minimum breaking pressure of the board when struck in the middle from my results of striking the top half off.

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