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Breaking bricks with fists

  1. Aug 30, 2007 #1
    Why does it hurt more when the break fails?

    I remember some guys explaining, that if the fist doesn't exert enough force on the brick for it to break, then the brick exerts more force on the fist. That makes intuitively sense as long as you don't know anything about Newton's laws. The forces should, of course, be equal in any case.

    That phenomenon makes sense when you think about it in terms of energy, since you might think that the kinetic energy of the fist either goes to the brick or the bones, but I have difficulty in seeing how the details go there. Why should the energy go to one place instead of another?

    Another direction: Is this original claim correct, or only a myth? I don't have personal experience with failed breaks. I know that if I hit a brick wall, the break will fail (or perhaps I succeed in breaking my fist), and it will hurt, but that's different. If the brick is placed so that it could be broken easily, and I then fail, the failure occured because of a weak punch, so will it still hurt?
     
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  3. Aug 30, 2007 #2
    Related question: Is it possible to have such brick, fist and direction of hit, that a sufficiently hard punch breaks the brick, and weak punch breaks the fist? To me this sounds like an impossibility, but again, I can hear martial arts guys talking about this.
     
  4. Aug 30, 2007 #3

    FredGarvin

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    It hurts more because one is feeling the entirety of the force against their fist. When the break works, the actual reaction force, the force you feel, is much less.
     
  5. Aug 30, 2007 #4
    Isn't that same thing as saying, that less force can break the brick, when more force doesn't break it?
     
  6. Aug 30, 2007 #5

    Gokul43201

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    Fred, this can't be true. If you apply less force than the breaking force, the reaction force is equal to the applied force. If you exert any force greater than the breaking force, the reaction force is equal to the breaking force until the instant the brick breaks.

    My guess is that when the brick doesn't break, the strike was not directed at the middle of the brick, where it takes least force to reach the breaking stress. Instead it was directed at a point where the required force to make the breaking stress is noticeably greater than at the center.
     
    Last edited: Aug 30, 2007
  7. Aug 30, 2007 #6
    On the other hand, the total impulse on the fist is greater when the break fails. If that's relevant.
     
  8. Aug 30, 2007 #7
    Striking the brick at its center allows more displacement, so for a given energy it requires less force overall.

    A symmetric, central hit minimizes lateral forces.

    Also, it seems self-fulfilling that one is more probable to break an object (by using less force) with an efficient blow.

    The fist with force evenly distributed at the resonance of the brick can withstand better than force unevenly distributed at the resonance of breaking bones.
     
  9. Aug 30, 2007 #8
    Force can be approximated by observing how the momentum of your fist change over time.

    In a case where you fail to break the brick, your fist comes to an instant stop there by increasing the Force exerted.

    In a case where you break the brick, your fist still moves after the break of the brick there by allowing more time for the momentum to change.
     
  10. Aug 31, 2007 #9
    Let us denote that force with [tex]F_{\textrm{fail}}[/tex].

    But the force that I needed to break the brick was [tex]F_{\textrm{break}} > F_{\textrm{fail}}[/tex].
     
  11. Aug 31, 2007 #10

    Pythagorean

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    Well, first of all, the brick has all the resistance of whatever is bracing it against your punch so if the brick is being held up on stands, which are touching the ground, then when you punch the brick, the brick itself, the stands, and a large chunk of Earth's surface around the stands are all resisting you too.

    If you break the brick (by exceeding the bond energy that holds the molecules of the brick together) it becomes a lot of smaller pieces, and you're not being resisted by everything that's bracing the brick against your blow, since the pieces aren't attached to it anymore

    So you want to make sure whatever material you use, the bond energy between molecules is less then the energy it takes to break your knuckles when holding a solid fist.

    conjecture:

    Some energy is also absorbed into the breaking of the bonds between the molecules that make up the brick.
     
    Last edited: Aug 31, 2007
  12. Aug 31, 2007 #11
    It doesnot happen

    Hi
    If i am not wrong than the force exerted by hand on the brick would be equal to the force applied by the brick on the hand.So the question of experiencing more force than applied is not possible
     
  13. Aug 31, 2007 #12
    Correct.

    I started thinking, that in the break only the knuckles feel the force, but in a failed break the arm is stopped so the force is felt also behind the knuckles elsewhere in the arm, and that could have effect on how it feels. But I still cannot believe, that the knuckles could break as result of too weak hit.
     
  14. Aug 31, 2007 #13

    learningphysics

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    I believe it is a question of time... when you don't break the brick... you're still exerting a force on the brick, and hence the brick is still exerting a force on your hand. It takes time to realize that the break didn't work and then to relax your hand...

    When your strike breaks the brick, the reaction force lasts a much shorter time than when your strike doesn't.
     
  15. Sep 2, 2007 #14
    Could be so. I put that this way
    Is there any knowledge about how the bones in the arm break if they break when brick doesn't? I mean, what bones precisely break first, and where? The knuckles, or bones higher in the arm?
     
  16. Sep 2, 2007 #15
    I'm just gonna take a stab at this, but...

    If I had to guess I would say that in the even the brick doesn't break the brick bends minorly, a compression wave emanating from the middle bouncing off both ends of the brick and traveling back to your hand, and it snaps back to its original state, pushing energy back into your arm.

    If it does break the energy associated with the bend reverberates through the two halves of the block, which are no longer aligned with your hand in a way that when the crest of the compression wave reached the point of impact that the energy just bounces back the other way within the brick.

    There is also likely a biomechanical difference in your arm between a successful break, and a failed break. I'm not sure exactly what it would be, but I think its safe to say that the way in which the arm is tensed which allows it to be "hard" enough to break a brick properly in the first place, is also a better absorber of the shock associated with it. Obviously, if you fail a break that you have the strength to do, then the difference is in your arm, and an improper alignment of the bones in your arm, or the muscles tensed to move your arm, is to blame, which may make it hurt more.

    Which bones would break I would guess depends on where in your arm the fault lies for failing the break.
     
  17. Sep 2, 2007 #16

    FredGarvin

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    I think I did the engineering equivalent to mixing my metaphors. I need to be more careful. The force by itself is not the key to breaks. The key is energy (like Pythagorean mentioned). If you hit with a smaller force, but an equivalent amount of energy, the board will break. Good breakers will have very fast hands as well as a lot of force behind their breaks. That energy is going one of two places; it's going into the deformation and bond breaking in the target or it's going into the deformation and bond breaking in your bones.
     
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