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Board breaking

  1. Jul 5, 2012 #1
    Hi all,

    I have a general question about board breaking. This is not homework.

    Can anyone explain why it hurts more not breaking the board, compared to breaking the board. I know that there are already threads about this topic. However, I am confused about the different explanations:

    1. If the board doesn't break there is a very large decrease in speed, causing the resulting force to be much larger.

    2. No energy is used in deforming the board. Therefore are larger amount of energy is absorbed by the hand.

    Can anybody explain to me how these explanations can be linked, or if they are wrong. I'd really like to understand this in context of Newton's third law.
  2. jcsd
  3. Jul 5, 2012 #2
    Those seem like reasonable explanations...
    Just think about hitting a concete wall for example....

    Typically when you break something and your hand/fist continues in motion; you have not
    reached maximum reaction forces...as when you slam into concrete, for example, and it stops your hand almost instantaneously...except for the deformation of your hand tissue.

    Another way to view this is FT = mΔv.....The force F exerted by your hand over a time T
    equals the change in momentum [mΔv] of the board. For some fixed momentum change you impart to the board, as time gets smaller the force [ouch!!] gets proportionally bigger. The time T over which the force F is applied is really tiny when the board neither deforms nor breaks.
  4. Jul 5, 2012 #3

    Simon Bridge

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    Well, if your hands starts out at speed v and is stopped by the board (no breaking) then the board absorbs all the energy and your hand will feel like it met another fist+arm swinging at the same speed the other way.

    OTOH, if it starts out at speed v and breaks the board, then your hand will still be moving after the breaking with speed u < v ... so it is as if your hand had met another one going slower ... less hurt.

    But - the harder the board is to break the more damage to your hand.

    However, you can experience failing to break a board on day one, but then break it or one substantially the same the next day, and the second time hurts less than the first time. Presumably you hit harder the second time...

    There is another effect ... when you break a board, you (a) psych yourself up and (b) feel elated afterwards. Both these effects will dull the pain, by a range of mechanisms including distraction and the release of endorphins. By comparison, if you fail to break the board because you were nervous or uncertain in yourself, you'll over-experience the pain ... you didn't get the endorphins, the anxiety hightens your senses, and the supervisor (and observers) show disapproval.

    This is why the exercize is so useful at motivational seminars.
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