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Where am I wrong

  1. Jun 6, 2003 #1


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    I'm confused w/ whether or not I want to or not want to maintain contact w/ something when I strike it to get the most affect. For example they say when you're hitting a baseball or a golf ball to maintain contact as long as possible (the 'follow through') to drive it as far as you can. However, when trying to karate chop boards in half or trying to knock-out somebody w/ a punch they say to minimize the contact time. Both are based on Force*time=change in momentum.

    It seems like the 2 contact time applications are contradictory where am I going wrong?
  2. jcsd
  3. Jun 6, 2003 #2


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    Science Advisor

    Greetings E8 !

    When you strike a ball you need to maximize
    the total momentum on it.
    Momentum is p = m * v , where m is mass and
    v is velocity. The total momentum on the ball
    is the sum of all the momentum transfered to it
    during the time of the interaction and so as
    long as your club still has some forward momentum
    left it's better to transfer as much of it as
    possible to the ball.

    When you try to break something you need to maximize
    your force or specific momentum. That is, you need a peak
    in momentum at some point thus resulting in a greater
    force F = m * a = m * v / t = p / t at that specific
    moment, to break whatever it is you're trying to break.

    Live long and prosper.
  4. Jun 6, 2003 #3


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    If what you're hitting is a house with a head and has a name like "Vini" - remain in contact for as short a time as possible. Then turn and run as fast as possible. :-)

  5. Jun 6, 2003 #4


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    Staff: Mentor

    Re: Re: Where am I wrong...

    There is a seeming paradox here: For example, though the tensile strength of steel is far higher than aluminum, steel requires less energy to break in an impact. There is a piece of equipment called a Charpy tester where you put small blocks of a given material into a device and swing an axe-like pendulum at them. When the pendulum breaks the block, it swings up to a specific height. The difference between the starting height and the new height is the kinetic energy required to break it - its the impact strength. Most steel is relatively brittle. When the pendulum hits it, it bends very little, concentrating the energy into a short, fast, hard force that easily snaps the block. Aluminum on the other hand is more ductile (softer). When the pendulum hits it, the energy is obsorbed when the aluminum bends. Because of this it takes many times as much energy to break an aluminum block in an impact than it does steel. Its acually a neat lab in materials science.
  6. Jun 6, 2003 #5


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    Science Advisor

    Re: Re: Re: Where am I wrong...

    I think that has to do more with elastic/non-elestic
    collisions and rigid/non-rigid bodies.
  7. Jun 7, 2003 #6
    When you hit a ball, you want to add as much momentum to it as possible -- as long as you can maintain contact, you are still pushing it, so it makes sense to follow through as long as you can. When you hit a person or a board, you're trying to hurt/break them, not push them: so you want to pack the energy of your blow into as short a time (and hence as large a force) as possible.
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